Heat oven to 350°F. In medium bowl, combine all meatball ingredients; mix well. Shape into 24 (1 1/2-inch) meatballs. Place in ungreased 13x9-inch (3-quart) baking dish.
Bake at 350°F. for 30 to 35 minutes or until no longer pink. Remove meatballs from baking dish; set aside. Drain dish.
Meanwhile, cook spaghetti to desired doneness as directed on package. Drain.
Place spaghetti in same baking dish. Stir in onions, spaghetti sauce and mushrooms. Top with cooked meatballs. Cover with foil.
Bake at 350°F. for 45 to 50 minutes or until thoroughly heated. Sprinkle with Parmesan cheese. Cover; let stand 5 minutes before serving.