Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside to cool slightly.
In a large mixing bowl, combine the chopped spinach, ricotta cheese, half of the mozzarella cheese, 1/4 cup of Parmesan cheese, garlic powder, onion powder, salt, and black pepper. Mix well until creamy and evenly blended.
Carefully stuff each cooked pasta shell with about 2 tablespoons of the spinach and ricotta filling.
Spread 1 cup of marinara sauce evenly on the bottom of a 9x13 inch baking dish.
Place the filled pasta shells in the dish, open side up. Pour the remaining marinara sauce over the top of the shells.
Sprinkle the remaining mozzarella cheese and Parmesan cheese over the sauce.
Cover the dish with aluminum foil and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Remove from the oven and let cool for a few minutes. Garnish with fresh basil leaves before serving.
Notes
Feel free to add more herbs or spices to the filling for extra flavor.