A delicious use for leftover chicken, turkey, or beef. Probably not the best choice for kids, but great for those who like their food on the spicy side.
Preheat the oven to 350 degrees F.
Melt the butter in a medium-size saucepan over medium heat. Add the onion and mushrooms and saute until limp, about 5 minutes. Blend in the flour. Stir in the chicken broth and continue to stir and cook until thickened. Remove from the heat and stir in the sour cream and chilies.
In a greased 2-quart casserole dish, layer the tortillas, chicken, sauce, and cheese, ending with a layer of cheese on top. Bake for 35 to 40 minutes, until heated through. Serve hot.