Cook spaghetti according to package directions. Meanwhile, melt butter in a large Dutch oven; saute green pepper and onion until peppers are crisp-tender. Stir in the chicken, mushrooms, pimiento, soup, milk, garlic powder and salt. Drain spaghetti and add to mixture; toss. Pour into a greased 13-in. x 9-in. baking dish. Cover and bake at 350° for 50-60 minutes or until hot and bubbly. Uncover; sprinkle with cheese; bake 10 minutes longer or until the cheese is melted. Yield: 10-12 servings.