Recipe: French Onion Soup Casserole (2024)

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Tami Weiser

Tami Weiser

Tami is a writer, recipe developer, culinary educator, and caterer. She is a graduate of the Professional Culinary Arts program at the Institute for Culinary Education (ICE) in New York.

updated Feb 3, 2020

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Recipe: French Onion Soup Casserole (1)

This French onion soup casserole takes everything your know about the homey dish and gives it a rich and luxurious update. Expect big flavor on this one!

Serves8

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Recipe: French Onion Soup Casserole (2)

If you’ve got leftover caramelized onions, or you are about to make a recipe that uses them, double up. And while you’re at it, grab that loaf of leftover bread because you are about to embark on a trip to France in one delicious vegetarian casserole based on the famous soupe à l’oignon gratinée.

Caramelized Onions Are a Leftover Rockstar

Caramelized onions are a result of that magical mixture of onions and time, cooked slow and on a low heat, so the onion’s natural sugars come forward and the onions become jammy. This recipe includes making caramelized onions for the base of the casserole, but I do my best to have some on hand almost all the time. Caramelized onions also keep, refrigerated, for about three days in a covered container, and can even be frozen.

A Mini Guide to Caramelized Onions

  • How To Caramelize Onions
  • Why You Should Freeze Caramelized Onions (and 3 Ways to Do It)
  • 5 Mistakes to Avoid When Making Caramelized Onions

Stock: The Foundation Flavor in a Vegetarian Casserole

To stock or not to stock? That is the question raised by so many chefs and cooks when it comes to French onion soup’s base. Most onion soups are made with a beef stock, made from browned bones and plenty of vegetables and cooked ad infinitum. This recipe creates its own vegetable stock as you cook the ingredients. It takes less than half an hour and works as a wonderful, supportive base for the incredible caramelized onions. If you start the onions from scratch, assume it’s about an hour of cooking time.

The Joys of Day-Old Bread

Second-day bread is a kitchen wonder. This recipe puts that loaf of bread to full use — creating a mini-grilled cheese sandwich — all right on top of the casserole to soak into the deliciousness below.

Comments

This French onion soup casserole takes everything your know about the homey dish and gives it a rich and luxurious update. Expect big flavor on this one!

Serves 8

Nutritional Info

Ingredients

For the stew:

  • 8 tablespoons

    (1 stick) unsalted butter, divided

  • 7

    large yellow onions (5 to 5 1/2 pounds total), halved and cut into 1/4-inch-thick slices

  • 4

    large shallots (about 8 ounces), peeled and cut into small dice

  • 4 teaspoons

    kosher salt, divided

  • 1 teaspoon

    freshly ground black pepper, divided

  • 6 cloves

    garlic, minced

  • 5 stalks

    celery (about 12 ounces), including leaves, cut into small dice

  • 5 carrots

    (about 8 ounces), peeled and cut into small dice

  • 6 cups

    cremini and/or button mushrooms (about 1 1/2 pounds), sliced thinly

  • Cooking spray

  • 1/4 cup

    all-purpose flour

  • 1 cup

    dry Riesling wine

  • 6 to 8

    fresh sage leaves, minced, or 2 teaspoons dried sage

  • Leaves of 5 sprigs fresh thyme, finely chopped

  • 1

    (15-ounce) can white beans and their liquid (optional)

For the topping:

  • 1

    large baguette (about 1 pound), cut crosswise into 24 slices

  • 12 ounces

    Gruyere, Comté, or Raclette cheese, shredded

  • 4 tablespoons

    unsalted butter

Instructions

Make the stew:

  1. Heat 4 tablespoons of the butter in a large, heavy-bottomed pot or Dutch oven over medium-high heat until melted and just beginning to foam.

  2. Add the onions, shallots, 1 teaspoon of the salt, and 1 teaspoon of the pepper. Cook, stirring occasionally, until the onions are translucent and begin to brown on the edges, 5 to 7 minutes. Reduce the heat to low (or the lowest your stovetop will go) and cook, stirring occasionally and adjusting the heat as needed, until the onions are completely softened and caramel brown in color, about 30 minutes.

  3. Add the garlic, celery, carrots, and mushrooms (add the mushrooms in batches if they do not all not fit into your pot at once, as they will cook down and reduce in volume), and stir to combine. Once you have added all the mushrooms, cover and cook 20 minutes more.

  4. Arrange a rack in the middle of the oven and heat to 375°F. Coat a 9x13-inch baking dish with cooking spray and set aside.

  5. When the vegetables are ready, transfer them with a slotted spoon to a large bowl and set aside. Pour the remaining vegetable cooking liquid into another bowl or a measuring cup with a spout; set aside.

  6. Return the pot to high heat, add the remaining 4 tablespoons of butter, and cook until it is melted and begins to foam. Add the flour, whisk to combine, and cook until it is golden-blonde in color, about 2 minutes. Add the wine and whisk to combine.

  7. Add the reserved vegetable cooking liquid and whisk to combine. Add the sage, thyme, and remaining 3 teaspoons salt and whisk to combine. Add the reserved cooked vegetables and beans and their liquid, if using, and stir to combine. Pour the mixture into the casserole dish and spread into an even layer; set aside.

Make the crouton topping:

  1. Arrange 12 of the bread slices on top of the casserole in a single layer, pushing them down gently into the liquid and vegetables. Sprinkle with about half of the cheese. Arrange the remaining 12 slices of bread over the cheese in a single layer, then sprinkle the remaining cheese on top. Melt the butter in the microwave, then drizzle over the casserole.

  2. Bake until the cheese is melted and the casserole is bubbly, 20 to 22 minutes. Serve immediately.

Recipe Notes

Make ahead: This recipe can be broken into parts and cooked over several days: Make the onions one day, cook it with the vegetables the next, and make the sauce and put the casserole together on the final day. Reheat the vegetable mixture before finishing the casserole.

Slow-cooker vegetable preparation: You can also make this recipe a second way, as a make-ahead, if you are short on time. Prepare the onions as written, but then transfer them and all the vegetables to a slow cooker, stir, and set at low or medium-low and cook for 3 to 4 hours. Then scoop the vegetables out, make the roux (flour and butter mixture) on the stove, add the wine and vegetable cooking juices to the pan to make the sauce, and add the cooked vegetables as written.

Storage: This dish is best eaten fresh, as the bread absorbs liquid as it sits and the casserole will get soggy. If you have any leftovers, they can be stored in an airtight container in the refrigerator for up to 3 days.

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Casserole

Recipe: French Onion Soup Casserole (2024)

FAQs

How do you deepen the flavor of French onion soup? ›

Olive oil – Add this when carmelizing the onions. Fresh thyme – This infuses the broth with fresh herby flavor and is scooped out just before the bread and cheese are added. Dry white wine – I use sauvignon blanc to deglaze the pan. Brandy – This enhances the other flavors.

What is the difference between onion soup and French onion soup? ›

What is the Difference Between Onion Soup and French Onion Soup? French onion soup is always made with beef stock. Other onion soups can be made with chicken, vegetable, or beef stock. Additionally, French onion soup typically contains wine or sherry, which isn't typical or necessary in other onion soups.

Should you cook onions before putting in casserole? ›

To prevent a soupy casserole, be sure to fully thaw any frozen vegetables in a colander over a bowl, and pat dry, squeeze excess moisture out of greens like spinach or chard, and quickly precook watery fresh vegetables like onions, mushrooms or squashes just until they release their moisture.

What broth is French onion soup made of? ›

What Is French Onion Soup? French onion soup, or soupe à l'oignon gratinée, is a classic French dish made with caramelized onions and beef stock or broth. The soup is topped with bread and cheese, then gratinéed to rich, gooey perfection. It's traditionally served in a ramekin or a ceramic crock with a handle.

How to doctor up canned French onion soup? ›

Some fresh rosemary or thyme brings a woody, peppery taste to the soup, complementing the umami flavor of the beef stock. With a splash of balsamic vinegar and green onions or fresh basil as a garnish, the canned soup is transformed from a grocery store original into a gourmet dish.

Why do you put baking soda in French onion soup? ›

The caramelized onions for this soup cook more quickly due to a pinch of baking soda but still have that deep, slow-cooked flavor. Topping the soup with hot, cheesy toasts prepared while the soup simmers, eliminates the need to bake the soup in specialty French onion soup bowls.

Why is French onion soup so expensive? ›

First, French onion soup calls for a ton of onions. While onions aren't expensive, there's a lot of labor involved in halving, peeling, and slicing 10, 12, 50, or one billion onions (I don't know how much soup you make).

Why does French onion soup taste so good? ›

French onion soup is the ultimate comfort food. Onions get slowly cooked until sweet and caramelized, then simmered in rich broth until they're practically falling apart. To finish it off, toasted bread is added to give it that lovable crisp-gone-soggy texture and a generous amount of Gruyère cheese is melted on top.

How can I thicken my French onion soup? ›

The easiest way to thicken your French onion soup is with flour. You can also use cornstarch if you don't have any flour. Remember to never add the flour or cornstarch directly to the soup – otherwise you run the risk of it clumping.

Should celery be sautéed before adding to soup? ›

While certain vegetables can work just fine added directly to simmering soups and stews (say, carrots and celery), other vegetables (onions, garlic, and the like) will almost always need at least a brief sweat in a fat-based liquid before adding the remaining ingredients.

Which onions are best for casseroles? ›

Yellow onions are perfect for stews, soups, barbecues, roasts and sauces. Avoid eating them raw as they have a strong flavour.

Should you cover casserole when baking? ›

Generally, casseroles with grains, rice or pasta that will cook during the baking process are usually covered, for at least part of the time. Casseroles made of cooked ingredients are usually baked uncovered. If you like a crisper, browner top, be sure the casserole is uncovered for at least part of the bake time.

What pairs well with French onion soup? ›

This recipe is very convenient as it takes only 10 minutes to prepare, allowing you to focus on making the soup.
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Jan 16, 2024

What is the bowl called for French onion soup? ›

restaurants serve French onion soup in a Lion Head soup bowl made of porcelain, because of the style and also, because it is an oven-safe bowl.

What is Panera French onion soup made of? ›

Sweet caramelized onions in a savory broth with a sherry wine vinegar gastrique, sea salt, topped with cheese and black pepper and sea salt croutons.

What to add to bland French onion soup? ›

Fix a bland onion soup by using supermarket broth and mild white onions, tasteless oil for sauteeing, stale plain white bread instead of croutons, and mild white cheese. Start by reducing the broth. Simmering out some water will intensify the flavor of the broth. Once you get it to the right point, add some salt.

Why is my French onion soup tasteless? ›

Lack of seasoning: Seasoning is crucial in any dish, and French onion soup is no exception. If you don't add enough salt, pepper, or other herbs and spices, the soup may taste bland.

How do you add depth of flavor to soup? ›

"If your broth is lacking in savory richness, try adding roasted onion, tomato paste, mushrooms, seaweed, soy sauce, or miso. These ingredients add umami flavor and depth to broth," she says. The choice of ingredient depends on the recipe, though.

How do you make soup taste stronger? ›

You can extract a clean, strong broth from a combination of water and several pantry ingredients. It's all about layering powerful flavor-enhancers that you probably already have on hand—bacon, tomato paste, herbs, peppercorns, a Parmesan rind, and, of course, kosher salt.

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