Cook spaghetti according to package directions. Meanwhile, in a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Stir in the taco seasoning, spaghetti sauce, mushrooms and olives. Drain spaghetti; stir into the beef mixture. Transfer to a greased shallow 3-quart baking dish; sprinkle with cheese. Bake, uncovered, at 350° for 25-30 minutes or until heated through. Serve with lettuce, tomatoes, sour cream and salsa.