Preheat your oven to 375°F (190°C) and lightly grease a large baking dish.
Cook the pasta shells according to package instructions until al dente. Drain and set aside.
In a large bowl, combine ricotta cheese, Parmesan, chopped spinach, egg, garlic powder, salt, and black pepper until well mixed.
Fill each shell with the ricotta and spinach mixture, then place in the prepared baking dish.
Pour marinara sauce over the filled shells, then sprinkle with shredded mozzarella cheese.
Cover with foil and bake for 20 minutes. Remove the foil and bake for an additional 5 minutes, or until the cheese is bubbly and golden.