Preheat oven to 375°F (190°C). Cook manicotti shells in boiling salted water until just al dente, about 6-7 minutes. Drain and lay out on a baking sheet to cool.

In a large skillet over medium heat, brown ground beef, Italian sausage, and diced onion, breaking up the meat, until well browned, about 8 minutes. Drain excess fat, leaving about 1 tablespoon.

Add minced garlic to the meat and cook for 30 seconds until fragrant. Stir in marinara sauce and simmer for 10 minutes to meld flavors.

In a large bowl, combine well-drained ricotta, 1 cup mozzarella, Parmesan, egg, dried basil, oregano, salt, and pepper. Mix until smooth and creamy.

Spread half of the meat sauce evenly in the bottom of the baking dish.

Using a teaspoon or piping bag, fill each manicotti shell with the cheese mixture. Arrange filled shells seam-side down in the baking dish.

Pour remaining meat sauce over the arranged shells. Sprinkle remaining 1 cup mozzarella on top.

Cover with foil and bake for 30 minutes. Remove foil and bake an additional 10 minutes until cheese is bubbly and golden.

Let the baked manicotti rest for 5 minutes before serving to allow filling to set.