Preheat the oven to 350°F (175°C).
Boil a large pot of water, adding a pinch of salt. Cook the pasta shells according to package instructions until al dente. Drain and set aside.
In a large mixing bowl, combine the chopped spinach, ricotta cheese, half of the mozzarella cheese, Parmesan cheese, egg, garlic powder, onion powder, salt, and pepper. Mix until well combined.
Fill each cooked pasta shell with the spinach and ricotta mixture, carefully placing the filled shells in a greased 9x13 inch baking dish.
Pour the marinara sauce evenly over the stuffed shells, ensuring they are well-coated. Sprinkle the remaining mozzarella cheese on top.
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 slightly golden.
Remove from the oven and let it cool for a few minutes before serving.