Preheat oven to 375°F (190°C). Cook pasta shells in salted boiling water until al dente. Drain and set aside on a tray to cool.
In a skillet, brown the ground beef until cooked through. Drain excess fat and set aside to cool slightly.
In a large bowl, mix cooked beef with ricotta, 1 cup mozzarella, Parmesan, egg, garlic, Italian seasoning, salt, and pepper.
Spread 1 cup of marinara in the bottom of a 9x13-inch baking dish. Fill each pasta shell with beef mixture and place in dish.
Top shells with remaining marinara and 1 cup mozzarella. Cover with foil and bake 25 minutes. Uncover and bake 10 more minutes.
Let rest 10 minutes before serving. Garnish with parsley if desired.
