In a large skillet, cook beef, onion and green pepper over medium heat until meat is no longer pink. Add tomatoes, water and chili powder. Cover and simmer for 30 minutes. Add the corn, peas, mushrooms, salt and pepper. Stir in spaghetti. , Layer half of the mixture in a greased 4-qt. baking dish. Sprinkle with 1 cup cheese; repeat layers. , Bake, uncovered, at 350° for 20 minutes or until heated through.