Add the flour to the remaining juices and stir to form a roux. Cook until bubbly. Slowly add the chicken broth and cook until thickened.
Remove from the heat and add the cream, sherry Parmesan, and nutmeg. Stir until the cheese melts.
Add the chicken and reserved mushrooms and stir well. Combine with the cooked spaghetti and place in a greased 3-quart baking dish. Melt 2 tablespoons of butter and toss with the bread crumbs. Sprinkle over the casserole.
Bake at 375 degrees F. for 25 to 30 minutes.