Cook the Pasta Shells: Bring a large pot of salted water to a boil. Add the pasta shells and cook until just al dente according to the package instructions. Drain and rinse under cold water to stop the cooking process. Set aside.

Prepare the Ground Beef Filling: Heat a skillet over medium heat. Add the ground beef and cook until browned, breaking it up into small crumbles. Add the chopped onion and minced garlic, cooking until softened. Drain any excess grease. Stir in 1 cup of marinara sauce, then remove from heat and let it cool slightly.

Mix the Cheese Filling: In a large mixing bowl, combine ricotta cheese, 1 cup mozzarella cheese, Parmesan cheese, egg, Italian seasoning, salt, and pepper. Mix until smooth and well combined. Fold in the cooled ground beef mixture.

Stuff the Shells: Preheat your oven to 375°F (190°C). Spread 1 cup of marinara sauce evenly over the bottom of a 9×13-inch baking dish. Stuff each pasta shell with the beef and cheese mixture and place them in the dish in a single layer.

Top with Sauce and Cheese: Pour the remaining marinara sauce evenly over the stuffed shells. Sprinkle the top with additional shredded mozzarella cheese.

Bake the Dish: Cover the dish with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.

Serve and Garnish: Remove from the oven and let the dish rest for a few minutes. Garnish with fresh parsley if desired and serve hot.