Cook manicotti according to package directions.
Remove manicotti from boiling water with tongs and place cooked shells on top of waxed paper to drain and cool.
Meanwhile, in a large skillet, cook beef and onion over medium heat until beef is no longer pink.
Use a hamburger chopper to break meat up into small pieces while frying.
Drain any grease.
Stir in the spaghetti sauce.
Spread half of the mixture into a greased 9x13” glass baking dish.
Stuff each manicotti shell with a piece of string cheese.
(Read my note below).
Place manicotti shells over top of the meat sauce; top shells with the remaining meat sauce.
Sprinkle with Italian seasoning, if desired.
Cover casserole dish with foil and bake at 350º for approximately 25-30 minutes, or until casserole is heated through and bubbly.
Sprinkle with Mozzarella cheese or Italian cheese blend.
Bake, uncovered for about 5-10 minutes, or until cheese is melted.
Serve immediately.