Preheat the oven to 375°F (190°C). Cook the manicotti shells according to package instructions until al dente. Drain and set aside to cool slightly.

In a large skillet, heat the olive oil over medium heat. Add the diced onion and cook until translucent, about 3-4 minutes. Add the minced garlic and ground beef, cooking until the beef is browned and cooked through. Drain excess fat if necessary.

Stir in half of the marinara sauce, oregano, basil, salt, and pepper. Remove from heat and let cool slightly.

In a mixing bowl, combine the ricotta cheese, half of the mozzarella cheese, Parmesan cheese, egg, and a pinch of salt and pepper. Mix until well combined.

Carefully stuff each manicotti shell with the ricotta mixture using a spoon or a piping bag.

Spread a thin layer of the remaining marinara sauce in the bottom of a 9x13-inch baking dish. Place the stuffed manicotti on top of the sauce in a single layer.

Pour the remaining marinara sauce over the stuffed manicotti, then sprinkle the remaining mozzarella cheese on top.

Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.

Let cool for 5 minutes before serving. Garnish with fresh basil leaves if desired.