Preheat Oven:Preheat your oven to 375°F (190°C).
Cook Pasta Shells:Bring a large pot of salted water to a boil. Cook the jumbo shells until al dente according to package instructions. Drain and set aside.
Prepare Beef Filling:In a skillet over medium heat, heat olive oil. Add chopped onion and cook until translucent, about 3-4 minutes. Add garlic and cook for 30 seconds. Stir in ground beef and cook until browned. Drain excess grease and remove from heat.
Mix Cheese Filling:In a large bowl, combine the cooked ground beef mixture, ricotta cheese, mozzarella cheese, Parmesan cheese, egg, Italian seasoning, salt, and pepper. Mix well.
Stuff the Shells:Fill each cooked pasta shell generously with the beef and cheese mixture.
Assemble Dish:Spread 1 cup of marinara sauce evenly on the bottom of a 9x13-inch baking dish. Arrange the stuffed shells in a single layer. Pour the remaining marinara sauce over the shells and sprinkle extra mozzarella on top.
Bake:Cover the dish with foil and bake for 25 minutes. Remove the foil and bake an additional 10 minutes until the cheese is bubbly and golden.
Serve:Garnish with chopped parsley and serve hot.