Heat the oven to 425°. Butter a 2 1/2- to 3-quart baking dish.
Cook spaghetti or linguine noodles according to package directions. Meanwhile, Meanwhile, in skillet, melt 4 tablespoons butter over medium-low heat; sauté sliced mushrooms until golden. Transfer to a bowl and set aside. Deglaze the pan with sherry on low heat. Cook and stir until almost all the liquid has been reduced. To the pan add 2 tablespoons of butter and the onion. Saute onions until tender, add the garlic and cook an additional 2-3 minutes longer. Add the diced bacon and heat through.
In a saucepan, melt 4 tablespoons butter; stir in flour. Stir and cook for 3 minutes, add cream and chicken stock, stir and cook until thick and smooth. Add chicken, and cooked mushrooms; heat through. Place noodles or spaghetti in a buttered baking dish; pour on sauce.
Combine Parmesan cheese and bread crumbs. Sprinkle over the top of the casserole.
Bake in a 400 to 425 degree oven for 15 to 20 minutes, or until hot and bubbly.
Tip: If you don't want to use cooking sherry, you can substitute with 1/4 cup apple juice and 1 tablespoon red wine vinegar.
Recipe by About a Mom at https://www.aboutamom.com/muellers-pot-sized-pasta-chicken-tetrazzini-recipe/