Preheat oven to 350°. Cook spaghetti according to package directions for al dente; drain. , In a large bowl, combine soups, sour cream, milk, 2 tablespoons butter and seasonings. Add cheeses, celery, onion and mushrooms. Stir in chicken and spaghetti., Transfer to a greased 3-qt. baking dish (dish will be full). Combine cornflakes and remaining butter; sprinkle over the top. , Bake, uncovered, 45-50 minutes or until bubbly.