Remove mushroom stems; set caps aside. Chop stems; set aside. In a skillet, cook sausage over medium heat until no longer pink; remove with a slotted spoon. In the drippings, saute onions and mushroom stems. Stir in spaghetti sauce and sausage. Stuff into mushroom caps. Sprinkle with cheese. Place in a greased 15-in. x 10-in. x 1-in. baking pan. Bake at 350 degrees for 12-15 minutes or until cheese is melted.