Preheat your oven to 350°F (175°C).
In a large pot, cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside to cool.
In a mixing bowl, combine the ricotta cheese, chopped spinach, ½ cup of the mozzarella cheese, Parmesan cheese, egg, garlic powder, salt, and black pepper. Mix well until fully combined.
Carefully fill each pasta shell with the spinach and ricotta mixture using a spoon.
Spread 1 cup of marinara sauce evenly in the bottom of your baking dish.
Place the filled pasta shells in the baking dish, open side up.
Pour the remaining marinara sauce over the stuffed shells, ensuring they are well covered.
Sprinkle the remaining mozzarella cheese on top of the shells.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
Let the dish cool for a few minutes before serving.