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? ›

A few sprigs of thyme and a bay leaf elevate the soup even more, but I take it a step further, adding a splash of fish sauce for complexity and depth—don't worry, it won't taste fishy—and a hit of cider vinegar to balance some of that oniony sweetness.

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.

How can I thicken my French onion soup? ›

Flour: a little all-purpose flour will help to thicken the soup. You can feel free to use gluten-free flour. Dry white wine: this helps to deglaze the pan and add a boost of flavor. Beef broth: I recommend a good quality beef broth or beef stock for the best flavor.

What spices add depth to soup? ›

Herbs and sources add flavor, aroma, and intensity to the soup broth. You can pick fresh or dried herbs like basil for tomato-based soups or fresh parsley for clear broths. You may also add more spices like turmeric, ground ginger, ground paprika, or nutmeg for a touch of spice and color to your soup broth.

How do you make soup taste stronger? ›

Use fresh vegetables, high-quality meats or broth, and fresh herbs to add depth of flavor. Add acidity: A squeeze of lemon or lime juice, a dash of vinegar, or a splash of wine can brighten the flavors of a soup and make it taste more complex.

Should you caramelize onions for French onion soup? ›

The key to good French onion soup is to cook the onions so long that they threaten to melt into a viscous, dark brown paste, à la Marmite. As has already been extensively reported in Slate, there are no shortcuts when you're caramelizing onions; it always takes at least an hour, usually longer.

Why do you put lemon juice in soup? ›

But as the publication explains, lemon juice adds more than just tartness. For chicken soup in particular, lemon juice balances out the more rich and savory flavors, adding an overall brightness to the taste.

Why does my French onion soup taste burnt? ›

If the heat is too high, the onions will burn. If you don't stir often at beginning and almost constantly toward the end, the onions will burn. If one little bit burns the whole batch needs to be tossed. Otherwise the soup ends up tasting like burnt onion.

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.

What is the best wine to use in French onion soup? ›

Wine is optional to deglaze the pan and add flavour to the soup. I recommend a dry white or red wine, such as a good quality Pinot Grigio/Gris, Sauv Blanc, Sémillon, Chardonnay, Merlot or Pinot Noir. Generally, if you can't drink it, don't cook with it.

Why do you add flour to French onion soup? ›

Onion soup, at its very essence, is nothing more than onions and water boiled together. Period, done, finished, everything added from that point on is pure opinion. Caramelizing the onions brings out sugars, and makes a more luxurious silky and sweet soup, adding flour gently thickens and provides body.

What is the etiquette for eating French onion soup? ›

With such a soup, a knife is your friend, not your enemy. Using both the soup spoon and the knife, push the baked cheese against the rim of the cup or bowl and cut it gently with the knife. Once you've broken the cheese barrier, you can use the knife again to nudge bits of cheese onto the spoon.

Is French onion soup good for your stomach? ›

French Onion Soup is a classic soup recipe that contains a clear broth base, which can help rehydrate your body and simultaneously ease stomach aches. Not to mention, it pairs perfectly with a crusty baguette or a handful of crunchy crackers.

How many days is French onion soup good for? ›

Leftover French onion soup can generally be stored in the refrigerator for about 3 to 4 days. Make sure to transfer it to an airtight container to maintain freshness and prevent odors from affecting the soup. If you don't plan to consume it within this timeframe, consider freezing the soup for longer storage.

How do you doctor up canned French onion soup? ›

Infusing the soup with a richer taste is as easy as quickly sauteing crushed garlic before pouring the soup in. Some soft, buttery garlic sauteed in earthy olive oil is the perfect addition to enrich canned French onion soup.

How do you get the depth of flavor in vegetable soup? ›

A squeeze of citrus, a dollop of yogurt or a drizzle of cream, a dusting of cheese or a good chile powder, a sprinkling of some herbs or croutons — all can take a perfectly fine soup into the realm of the delectable. Even a spiral of good olive oil to finish and some coarse sea salt can do wonders.

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