Cook manicotti according to package directions in salted water.
While pasta is cooking, cook beef and onion over medium heat in a large skillet until meat is no longer pink; add garlic and cook for about 1 minute. Drain meat. Stir in spaghetti sauce. Spread half of the meat sauce into a greased 9x13 inch baking dish.
Drain manicotti. An 8 ounce package has 14 shells but I could only fit 12 in my baking dish and 2 of the shells ripped. Stuff each shell with a piece of string cheese. You could even use 2 pieces of string cheese but we thought one was plenty once it melted. Arrange manicotti over meat sauce. Place over meat sauce; top with remaining sauce. Cover with foil and bake at 350° for 25-30 minutes or until heated through.
Sprinkle with mozzarella cheese and bake for 5 minutes or until the cheese is melted. Sprinkle with grated parmesan cheese and garnish with basil if desired.
Source: slightly adapted from Taste of Home: http://www.tasteofhome.com/recipes/easy-to-stuff-manicotti