French Onion Soup Casserole

This savory French Onion Soup Casserole has all the goodness and flavors of your favorite French onion soup. Sweet caramelized onions, Gruyère cheese, fresh thyme leaves and sourdough French bread. All layered together in a casserole, then soaked with a rich flavor broth.

4.75 from 12 votes

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Prep Time: 1

Cook Time: 55

Total Time: 1 55

Servings: -10

Calories: 606kcal

Author: Pat Nyswonger

Ingredients

1 large 24-ounce loaf French sourdough bread, pre-sliced
8 tablespoons 4 ounces unsalted butter, divided
2 tablespoons extra-virgin olive oil
5 pounds about 6 large yellow onions, thinly sliced
Sea salt
1/2 cup white wine
1/4 cup fresh thyme leaves
12 ounces Gruyère cheese grated (about 3 cups)
3 ounces Parmesan finely grated (about 1 1/3 cups)
5 to 6 cups beef stock
Freshly ground white pepper
2 tablespoons butter cut into small cubes

Instructions

Preheat the oven to 325°F
Cut the crusts off the bread slices then place the slices in single layers on 2 or 3 sheet pans. Toast the bread for 20 minutes, then flip slices, and rotate pans to ensure even toasting.
Toast until dried out and lightly golden, about 35-40 minutes total. Set aside.
Turn oven off until later.

While the bread is toasting:

Set a large Dutch oven or large pot over medium heat. Add 4 tablespoons butter and olive oil. When butter has melted, add onions and 1-1/2 teaspoons salt. Cook covered for 15 minutes, stirring occasionally with a wooden spoon and scraping up bits from the bottom.
Once the onions cook down they will release some liquid, remove lid and increase heat to medium high. Continue cooking and stirring occasionally until onions are golden brown, about 45 minutes total.
Turn off heat, add the wine and stir to deglaze the pan. Taste, and adjust salt as needed, stir in the fresh thyme leaves and set aside.
Place both cheeses in a medium bowl, and mix to combine.
Heat the oven to 400°F and adjust oven rack to center position.
Coat the inside of a deep 9-by-13-inch baking dish with oil spray. Line the bottom with a layer of toasted bread, breaking up pieces as needed to form a solid layer.
Spoon half the onion mixture evenly over the bread. Sprinkle with 1/3 of the cheese mixture, and season with pepper. Continue layering with bread, the remainder of the onions and another 1/3 of the cheese. Top off with a final layer of bread.
Ladle 3 cups of hot broth over the panade, then wait a minute, and allow the bread to absorb the liquid. Press down on panade with a metal spatula, add as much broth as the panade will absorb without overflowing.
Dot the top layer with remaining butter, then cover with a piece of oil-sprayed foil.
Place casserole dish on a baking sheet to catch any overflow, then transfer to the center rack.
Bake for 30 minutes, then remove foil and sprinkle with remaining 1/3 of cheese and return to oven for 15 minutes more. Bake until golden brown.
Remove casserole from the oven, and allow it to cool for 10 minutes before serving.

Notes

You may not need all of the bread slices. We used 10 slices total.
Turn the oven off after toasting the bread. Preheat to 400°f after the onions have cooked.
Allow the panade to rest for 10 minutes before serving.
Reheat any remaining broth and serve in the dish with each portion.