You Must Make This French Onion Soup Casserole

We know what’s on your mind. How does soup, usually a liquid, become a casserole, a solid, thick dish? French onion soup is definitely magic!

The honest answer is flour, which, if you think about it, has some magical qualities. Flour is the best thickener. It turns a soupy-ish consistency into something with a little more structure.

Think of a roux made by mixing flour and fat (usually butter) in equal amounts. Slowly, a liquid (usually milk, cream, or broth) is added, and things start to get creamy. Here’s the trick to this soup casserole: the base doesn’t get smooth in this order.

Beef broth and wine are all needed to make the base of traditional French onion soup. We’re also a big fan of tomato paste in this dish because it gives everything a complex, acidic boost and gives the broth the stickiness it needs. Also, the combination of dry red wine and nutty, fortified Sherry is delicious, but white wine works just as well.

In the first step of this recipe, caramelized onions are cooked in butter. Once those have reached their peak, we add flour, which acts like a magic thickener. Next come beef broth and wine, which are the usual suspects. But since we said flour, what used to be regular French onion soup is now a creamy, dreamy pot of caramelized onions and stock flavored with wine.

French Onion Soup Casserole

Course: SoupsCuisine: FrenchDifficulty: Medium


Prep time


Cooking time



Why stick to just one cozy culinary adventure when you can't decide between a bowl of soup and a casserole that slides out of the oven?
This French onion soup casserole is the best of both worlds because the most critical parts (sweet caramelized onions, bread, and cheese) still find a way to go together.
If you're cooking for someone who doesn't mind meat but prefers chicken to beef, the chicken stock will work just fine. The final product won't be as dark or fatty, but the handfuls of cheese will make up for that.

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  • 4 tablespoons Unsalted Butter

  • 3 Vidalia onions

  • 2 teaspoons of regular salt

  • 12 teaspoons black pepper

  • 2 dried bay leaves

  • 2 tablespoons fresh finely chopped thyme

  • 1 teaspoon of tomato paste

  • 4 tablespoons All-Purpose flour

  • 1 teaspoon Worcestershire sauce

  • 14 cups dry sherry for cooking

  • 14 cups of dry red wine

  • 6 cups of low-salt beef broth

  • 1 baguette

  • 2 teaspoons olive oil

  • 1 cup of shredded provolone piccante (More sharp profile than normal provolone cheese)

  • 1 cup of grated Gruyere

Make the CARAMELIZED onions

  • Cut the onions and thyme into small pieces.
  • Over medium-low heat, put the butter in a heavy pot with a wide bottom. Add the onions and garlic once it starts to foam and sizzle.
  • Cook, stirring every as frequently, for about 20 minutes, or until the onions start to get very soft and let out their juices.
  • Add one teaspoon salt, pepper, bay leaves, and one tablespoon of thyme. Cook occasionally for about 40 more minutes until the onions are fully caramelized and a deep golden brown.
  • While the onions are getting sweet, toast the slices of baguette. Turn the oven on to 350°F.
  • Spread the baguette pieces in a single layer on a baking sheet and drizzle olive oil on both sides.
  • Bake for 12 to 14 mins, turning once halfway through, until golden brown and toasted.
  • Put the bay leaves out of the pot and mix the tomato paste and flour. Turn the heat up to medium and stir the food now and then for 2 minutes.
  • Pour the Worcestershire, sherry, wine, and beef broth into the pan and use a wooden spoon to scrape up any bits stuck to the bottom.
  • Bring the soup to a boil and constantly stir for about 2 to 3 minutes, or until it thickens. Then, add the last teaspoon of salt.
  • Layer half of the toasted baguette slices and half of the onion soup in a 9x13-inch baking dish.
  • Spread the rest of the baguette slices and Provolone Piccante on top in an even layer.
  • Spread the grated Gruyere all over the top of the bread to cover it.
  • Cover the dish with aluminum foil and bake for 30 minutes.
  • Take off the foil and put the casserole under the broiler until the cheese bubbles and turns golden.
  • Divide the soup between bowls and sprinkle the tops with the last tablespoon of thyme. Bonjour, Monsieur Casserole!

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Can You Make French onion soup the day before?

Yes! Follow the recipe exactly up until the point where the soup is finished, but skip adding the cheesy croutons if you’re making French onion soup ahead of time. We strongly advise you to prepare French onion soup in advance since it tastes even better if you do. 

Can you overcook French onion soup?

Cooking them too quickly over a high heat risks burning them and instilling a scorched, bitter flavor to your soup, so be careful not to rush this step.

How long is French onion soup good for?

Refrigerate the soup (without the croutons) in a sealed container for 3 to 4 days.

What are the bowls called for French onion soup?

Traditional French onion soup is served in lionhead bowls, that are profound but not too wide porcelain bowls.