Preheat your oven to 350°F (175°C).
Cook the jumbo pasta shells according to the package instructions until al dente.
In a large mixing bowl, combine ricotta cheese, mozzarella cheese, Parmesan cheese, chopped spinach, egg, garlic powder, Italian seasoning, salt, and pepper.
Spoon the cheese and spinach mixture into each cooked pasta shell.
Spread a layer of marinara sauce in the bottom of a greased baking dish.
Arrange the filled shells over the marinara sauce in the baking dish.
Pour the remaining marinara sauce over the stuffed shells.
Sprinkle additional mozzarella and Parmesan cheese on top.
Cover the baking dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10–15 minutes until the cheese is bubbly and golden.
Allow the stuffed shells to cool slightly before serving.