Preheat the oven to 400°F. Grease a 13 x 9-inch baking dish.
Cook in a large pot of boiling salted water: spaghetti, macaroni, or egg noodles. Meanwhile, melt 6 tbsp. butter in a saucepan over medium heat.
Add and cook mushrooms, stirring until softened, about 5 minutes. Stir in flour until well blended.
Slowly add while whisking: chicken stock or broth; whole milk or half-and-half; and dry sherry (optional). Bring to a boil, reduce the heat, and cook until the sauce is thickened and smooth, about 5 minutes. Mix in chopped cooked turkey.
Drain the pasta and gently mix into the turkey mixture. Pour into the prepared dish.
Mix Parmesan and bread crumbs together in a small bowl. Sprinkle the mixture over the casserole and dot with remaining 2 tbsp. butter cut into small cubes.
Bake until the sauce is bubbling and the topping is golden brown, 25 to 35 minutes.