Cook the spaghetti according to package directions; drain and place in an ovenproof casserole dish.
Melt the butter in a frying pan over medium heat. Cook the celery, onions and mushrooms until tender, about 5 minutes. Add the soup, milk, sherry, cheese and season with salt and pepper. Cook over low heat until the cheese has melted, about 3 minutes.
Add the turkey to the sauce; pour over the spaghetti and stir well. Sprinkle with Parmesan cheese.
Bake in the preheated oven until bubbly and the cheese has melted, 20 to 25 minutes.