Preheat the oven to 375°F (190°C).

Bring a large pot of salted water to a boil. Add manicotti shells and cook for 6-7 minutes until al dente. Drain and cool.

In a skillet, heat and brown ground beef and Italian sausage with chopped onion for about 8 minutes. Drain excess fat.

Add minced garlic to the skillet and sauté for about 1 minute. Pour in marinara sauce and simmer for 10 minutes.

In a mixing bowl, combine ricotta, 1 cup mozzarella, grated Parmesan, egg, basil, oregano, salt, and pepper. Mix until smooth.

Spread half the meat sauce in a 9x13-inch baking dish. Fill manicotti shells with ricotta mixture and place seam-side down in the dish. Top with remaining meat sauce.

Sprinkle remaining mozzarella on top and cover with aluminum foil. Bake for 30 minutes, then uncover and bake an additional 10 minutes.

Let rest for 5 minutes before serving.