Cook manicotti according to package directions. In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Stir in spaghetti sauce. Spread half of the meat sauce into a lightly greased 13 x 9 baking dish.
Drain manicotti; stuff each shell with a piece of string cheese. Place over the meat sauce; top with remaining sauce. Cover and bake at 350 for 30 minutes.
Sprinkle with mozzarella cheese. Bake uncovered for 5-10 minutes longer until cheese is melted.