Preheat your oven to 375°F (190°C).

Cook the jumbo pasta shells according to package directions until al dente. Drain and set aside.

In a large skillet, cook the ground beef, onion, and garlic over medium-high heat until the beef is browned. Drain off any excess grease.

In a large bowl, combine the cooked beef mixture with ricotta cheese, Parmesan cheese, egg, parsley, Italian seasoning, salt, and pepper. Mix well.

Spread 1 cup of marinara sauce in the bottom of a 9×13-inch baking dish.

Carefully stuff each cooked shell with the beef and ricotta mixture and arrange them in the baking dish.

Pour the remaining marinara sauce over the shells, then sprinkle with the shredded mozzarella cheese.

Cover with foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes or until the cheese is bubbly and lightly browned.