Prepare the filling: In a skillet, cook ground beef with chopped onion and garlic until beef is browned and onions are softened. Drain excess fat. Stir in Italian seasoning, salt, and pepper. Let cool slightly. Combine beef mixture with ricotta, mozzarella, Parmesan, parsley, and egg in a bowl. Mix well.
Prepare the sauce: In a small saucepan, warm marinara sauce with olive oil, oregano, and basil until heated through.
Stuff the manicotti: Carefully fill each uncooked manicotti shell with the beef and cheese mixture using a spoon or piping bag.
Assemble the dish: Spread a thin layer of marinara sauce in a 9x13-inch baking dish. Arrange stuffed manicotti on top and pour remaining sauce over them. Sprinkle shredded mozzarella and Parmesan over the top.
Bake: Cover with foil and bake at 375°F (190°C) for 25 minutes. Remove foil and bake for an additional 10 minutes until cheese is melted and bubbly.
Serve: Garnish with fresh basil or parsley and serve warm.