Preheat the oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish and set aside.

Cook the manicotti shells in a large pot of salted boiling water according to package directions, removing them 2 minutes before the recommended time. They should be al dente and slightly firm. Drain, rinse with cold water, and lay out on a lightly oiled surface to prevent sticking.

In a large skillet over medium heat, cook the ground beef and diced onion, breaking the meat into small pieces, until no longer pink. Add the minced garlic and cook for 1 more minute. Drain any excess grease and let the mixture cool for 5 to 10 minutes.

In a large bowl, combine the cooled meat mixture, ricotta, 1 cup of the mozzarella, Parmesan, egg, oregano, salt, and pepper. Stir until fully combined into a thick, cohesive filling.

Transfer the filling to a gallon-sized zip-top bag and snip one corner off. Pipe the filling into both ends of each manicotti shell until full. Alternatively, use a small spoon to fill the shells from both ends.

Spread about 1 cup of the marinara across the bottom of the prepared baking dish. Arrange the stuffed shells in a single layer over the sauce.

Pour the remaining marinara over the shells, ensuring every shell is fully covered. Scatter the remaining 1 cup of mozzarella evenly over the top.

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

Remove from the oven and let the dish rest for 5 to 10 minutes before serving to allow the filling to set.