Step 1

Preheat the Oven: Preheat your oven to 425°F (220°C) and butter a 9-by-13-inch or 3-quart baking dish.

Step 2

Cook the Mushrooms: In a large pot, melt 1 tablespoon of butter over medium-high heat. Add the sliced mushrooms and cook without stirring until browned on the bottom (about 2 minutes). Stir and continue cooking until browned all over (about 2-3 minutes). Remove the mushrooms to a plate.

Step 3

Cook the Onions: Add the remaining 4 tablespoons of butter to the pot and melt over medium-low heat. Add the thinly sliced onions and cook, stirring frequently, until they become dark golden (about 20-25 minutes).

Step 4

Combine Ingredients: Return the mushrooms to the pot, add the chopped thyme, kosher salt, and a few grinds of black pepper. Sprinkle in the flour and stir to coat the vegetables. Let it toast for a minute, then increase the heat, pour in the wine, and bring to a simmer. Add the beef broth and shredded chicken, returning to a simmer, and cook until thickened (about 3 minutes).

Step 5

Assemble the Casserole: Ladle enough of the onion-chicken mixture into the prepared baking dish to cover the bottom. Arrange half of the baguette slices in a single layer on top. Ladle half of the remaining onion-chicken mixture over the bread and push down on the baguette slices with the bottom of the ladle to submerge them. Sprinkle with 1 cup of Gruyère cheese and ¼ cup of Parmesan cheese. Top with the remaining baguette slices in a single layer, then ladle the remaining onion-chicken mixture on top, and sprinkle with the remaining 1½ cups of Gruyère and ¼ cup of Parmesan.

Step 6

Bake: Cover the casserole with foil, tenting it so it doesn’t touch the cheese. Place on a rimmed baking sheet and bake until bubbly (about 20 minutes). Uncover and bake until the top is browned and crusty (about 15 minutes more). Let sit for 10 minutes before serving.