Heat a pot of salted water to a boil, add manicotti shells and cook until shells are not quite cooked through, about 2-3 minutes underdone, then drain and run under cold water.
Cooked and drained shells. OR you can use dry shells but you will need to increase the bake time.
While the pasta is cooking, heat a pot and cook the onion and ground beef, breaking the beef up with the spatula as it cooks. Cook until the beef is no longer pink. Drain any fat and return the pot, with the beef, back to heat.
Preheat your oven to 180° C (350° F). Get out a 9x13 inch baking pan.
To the pot, add the marinara sauce, Italian seasoning, and garlic powder, stir together.
Spread half of the meat sauce in the baking pan.
Stuff each manicotti shell with 2 pieces of string cheese. Arrange all the shells in the pan in a single layer. If the string cheese is too much, pull one piece of cheese in half and stuff a shell with 1½ pieces of cheese. OR you can stuff the dry shells with 1½ pieces of cheese but the baking time will increase.
I stuffed the shells dry, arrange all the shells in the pan in a single layer.
Top the manicotti with the remaining sauce.
Top with Mozzarella cheese. Sprinkle with Italian seasoning as desired.
Bake for 30 minutes if using cooked pasta, or 50 minutes if using dry pasta, or until bubbly and starting to brown.
Garnish with parsley as desired, serve and enjoy.