Bring a large pot of salted water to a boil, add manicotti shells, and cook for 4-6 minutes until slightly softened. Drain and lay on a baking sheet to cool.

In a large skillet, heat olive oil over medium heat. Sauté coarsely chopped onion for 3-4 minutes until translucent. Add ground beef and cook 5-7 minutes until browned, seasoning with salt, pepper, and garlic.

In a large bowl, combine the sautéed beef and onion with ricotta, mozzarella, Parmesan, and parsley. Mix well to create the filling.

Using a spoon or piping bag, fill each manicotti shell with the filling. Set aside.

Preheat oven to 350°F. In a baking dish, pour a thin layer of marinara sauce, arrange stuffed manicotti, then cover with remaining sauce and sprinkle with mozzarella and Parmesan.

Cover the baking dish with foil and bake for 30-35 minutes, removing foil in the last 10 minutes for a golden top.

Cool for 5-10 minutes before serving, garnished with parsley.