Cook the Shells: Boil the jumbo pasta shells in salted water according to package instructions until al dente. Drain and set aside.
Prepare the Filling: In a mixing bowl, combine ricotta cheese, 1/2 cup of mozzarella, Parmesan cheese, egg, Italian seasoning, salt, and pepper. Mix until well combined.
Stuff the Shells: Preheat your oven to 375°F (190°C). Spread 1 cup of marinara sauce on the bottom of a baking dish. Fill each cooked shell with the cheese mixture and place them in the dish.
Add Sauce and Cheese: Pour the remaining marinara sauce over the stuffed shells and sprinkle with the remaining mozzarella cheese.
Bake: Cover with foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
Serve: Garnish with chopped basil if desired and serve hot.