Preheat your oven to 375°F (190°C).

In a large pot, bring salted water to a boil and carefully add your manicotti shells. Cook them for 6-7 minutes until they are al dente but not fully soft, then drain and lay them on a baking sheet to cool.

In a large skillet over medium heat, add ground beef and Italian sausage, stirring in diced onion. Cook for about 8 minutes, or until the meat is browned and the onion is translucent.

Sprinkle in minced garlic and sauté for another 30 seconds until fragrant, then pour in your marinara sauce. Let the mixture simmer for 10 minutes.

In a mixing bowl, combine ricotta, 1 cup of mozzarella, Parmesan, egg, fresh basil, oregano, salt and pepper. Mix until everything is well incorporated.

Spread half of the meat sauce evenly in a baking dish. Carefully stuff each manicotti shell with the cheesy mixture, and arrange them seam-side down in the dish.

Cover the stuffed manicotti with the remaining meat sauce and sprinkle the rest of the mozzarella cheese on top.

Cover the baking dish with foil and bake in the preheated oven for 30 minutes. After that, remove the foil and bake for an additional 10 minutes, or until the cheese is melted and lightly golden.

Once baked, allow your Stuffed Manicotti to sit for 5 minutes before serving.