Preheat your oven to 375°F (190°C) and bring a large pot of salted water to a rolling boil.

Cook the manicotti shells in boiling water for 6-7 minutes until just al dente.

In a large skillet, heat olive oil over medium heat. Brown ground beef, Italian sausage, and diced onion for about 8 minutes.

Add minced garlic to the skillet and sauté for 30 seconds. Pour in marinara sauce, let simmer for 10 minutes.

In a mixing bowl, combine ricotta cheese, 1 cup of mozzarella, Parmesan, egg, basil, oregano, salt, and pepper.

Spread half of the meat sauce in a baking dish, fill manicotti shells with cheese mixture, place in dish seam-side down.

Cover with remaining meat sauce and reserved mozzarella. Cover with foil and bake for 30 minutes, then uncover and bake for another 10.

Let rest for 5 minutes, then slice and serve warm.