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 9x13” baking dish.

Drain manicotti; stuff each shell with a piece of string cheese.

Place over meat sauce; top with remaining meat sauce.

Cover and bake at 350 degrees for 25-30 minutes or until heated through.

Sprinkle with mozzarella cheese.

Bake, uncovered for 5-10 minutes or until cheese is melted.