Cook manicotti 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 spaghetti sauce. Spread half of the meat sauce into a greased 13-in. x 9-in. 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 24-30 minutes or until heated through.
Sprinkle with mozzarella cheese. Bake, uncovered, for 5-10 minutes longer or until the cheese is melted. Yield: 6-8 servings.