In a large nonstick skillet, cook the beef, mushrooms and onion over medium heat until meat is no longer pink; drain. In a shallow 3-qt. baking dish coated with cooking spray, toss pasta, Parmesan cheese, green pepper, spaghetti sauce and beef mixture.
Cover and bake at 350° for 30 minutes. Uncover; sprinkle with mozzarella cheese. Bake 10 minutes longer or until cheese is melted. Let stand for 10 minutes before serving.