Preheat the oven to 375°F (190°C).
Cook the jumbo pasta shells according to package instructions, drain and set aside.
In a large mixing bowl, combine ricotta cheese, mozzarella cheese, Parmesan cheese, egg, parsley, garlic powder, Italian seasoning, salt, and pepper. Mix well.
Stuff each cooked pasta shell with the cheese mixture and place them in a baking dish.
Pour marinara sauce over the stuffed shells, making sure they are well covered.
Bake in the preheated oven for 30 minutes, until the sauce is bubbly and the cheese is melted and golden.
Garnish with additional parsley if desired and serve hot.