Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.

In a large pot, bring salted water to a boil, and add the manicotti shells. Cook until just al dente, about 7-9 minutes.

In a large skillet, cook the ground beef and Italian sausage over medium heat for 6-8 minutes, breaking it up until browned.

Add onion and garlic to the skillet with the browned meat, and sauté for 3-4 minutes until the onion is translucent.

Stir in oregano, basil, salt, and black pepper. Remove from heat and let cool slightly.

In a mixing bowl, combine ricotta, mozzarella, Parmesan, and egg. Mix until creamy, then fold in the meat mixture.

Fill each manicotti shell with the meat and cheese mixture and place them upright in the greased baking dish.

Pour marinara sauce over the stuffed manicotti, ensuring each shell is coated.

Top with remaining mozzarella cheese.

Cover with foil and bake for 25 minutes, then remove foil and bake for an additional 10-15 minutes.

Let the manicotti rest for 5 minutes before serving. Garnish with fresh parsley.