Preheat your oven to 375°F (190°C).

In a large pot, bring salted water to a rolling boil. Carefully add the manicotti shells and cook for about 6-7 minutes until al dente. Drain and rinse under cold water.

Heat a large skillet over medium heat, then add the ground beef and Italian sausage. Sauté for about 8 minutes. Add diced onion, and sauté until translucent. Add minced garlic in the last minute.

Stir in marinara sauce with the meat mixture and simmer for about 10 minutes.

In a mixing bowl, combine ricotta cheese, mozzarella, grated Parmesan, egg, basil, oregano, salt, and pepper. Mix until smooth.

Ladle half of the meat sauce into a prepared baking dish. Fill each manicotti shell with the cheese mixture and place in the dish. Pour remaining sauce over the shells.

Sprinkle additional mozzarella cheese on top. Cover with foil and bake for 30 minutes, then uncover and bake for another 10 minutes.

Let the manicotti rest for about 5 minutes before serving.