Cook manicotti according to pkg. directions. Meanwhile, cook beef and onion, drain. Stir in spaghetti sauce. Spread 1/2 meat sauce into greased 9x13 inch baking dish. Drain manicotti, stuff each shell with a piece of string cheese. Place over meat sauce. Top with remaining sauce. Cover and bake at 350 for 25 to 30 minutes until heated through. Sprinkle with mozzarella cheese. Bake uncovered, for 5 to 10 minutes or until cheese is melted.