Cook the pasta shells:
Bring a large pot of salted water to a boil. Cook the jumbo pasta shells according to package directions until al dente. Drain and set aside to cool slightly.

Cook the ground beef:
In a large skillet, cook the ground beef with diced onion over medium heat until browned, about 5-7 minutes. Drain excess fat. Add minced garlic and Italian seasoning, and cook for an additional 1-2 minutes. Season with salt and pepper, then remove from heat.

Make the cheese filling:
In a mixing bowl, combine the ricotta cheese, shredded mozzarella, Parmesan cheese, egg, and a pinch of salt and pepper. Mix well. Stir in the cooked ground beef mixture.

Preheat the oven:
Preheat your oven to 375°F (190°C).

Stuff the shells:
Spread 1 cup of marinara sauce evenly on the bottom of a 9x13-inch baking dish. Using a spoon, stuff each shell with the meat and cheese mixture. Place the stuffed shells in the baking dish, open side up.

Top with sauce and cheese:
Pour the remaining marinara sauce over the stuffed shells and sprinkle with extra shredded mozzarella cheese.

Bake:
Cover the baking dish with foil and bake for 25-30 minutes. Remove the foil and bake for an additional 5-10 minutes, until the cheese is melted and bubbly.

Garnish and serve:
Let the dish cool for a few minutes before garnishing with fresh basil or parsley. Serve hot.