​This is some GOOD stuff! I got the idea from Ree Drummond – the Pioneer Woman. I changed it up a bit. For instance, I used home-made chicken stock rather than store-bought beef broth. And we used large sweet onions and probably a pound of fresh white mushrooms which we sliced thick. Plus, I made up some extra roux and gravy at the end as I had a lot of liquid in the pan.

Ingredients

unsalted butter, plus more for the baking dish
16-ounce package white mushrooms, thickly sliced
4-5 large sweet onions, thinly sliced
fresh thyme, chopped or a bit of dried thyme
1/2 tsp.kosher salt
Black pepper, to taste
4 tbsp all-purpose flour
1 cup dry white wine
3 cup home-made chicken stock
2 – 3 cups cooked chicken thighs; chopped
3 bollios or 1 long baguette, sliced 1/2 inch thick
1/2 cup (or more) shredded gruyère cheese (about 10 ounces)
grated parmesan cheese

Directions

In 5 T of butter, caramelize four or five large sweet yellow onions with a sprinkle of salt. When they’ve browned a bit and most of the liquid has cooked off, remove onions to a plate and set aside. In the same skillet, cook the mushrooms until slightly browned.

Add the onions back to the skillet and sprinkle with flour. Stir to coat. Toast for a bit to cook off the flour some. Deglaze pan with some white wine. Then stir in chicken stock and chopped chicken meat. In separate skillet make up some more roux by cooking a few T of flour in butter. Stir in pan liquids from onion/mushroom mixture to make a gravy. In a squeeze, I supplement this with a cornstarch slurry made with the pan liquids.

Ladle onion/mushroom/chicken mixture into the baking dish to cover the bottom. Cover this layer of sauce with bread slices. Ladle sauce on the bread; then top with grated cheese. Repeat and then top casserole with cheese.

Cover the casserole with foil. Create a tent so foil does not touch the cheese. Place on a rimmed baking sheet and bake for about 20 minutes. Uncover and bake until the top is browned. About 10 or 15 minutes more. Let sit for 10 minutes before serving.