Boil a large pot of salted water and cook the jumbo pasta shells according to package instructions until al dente. Drain and let cool.
In a large bowl, mix together the ricotta cheese, chopped spinach, mozzarella cheese, Parmesan cheese, egg, garlic powder, onion powder, Italian seasoning, salt, and pepper until well combined.
Spread 1 cup of marinara sauce evenly in the bottom of a 9x13 inch baking dish.
Take each cooked pasta shell and carefully fill it with a generous tablespoon of the ricotta-spinach mixture. Place the filled shells in the baking dish.
Once all shells are filled and arranged tightly in the dish, pour the remaining marinara sauce over the top of the shells.
Sprinkle additional mozzarella cheese on top of the sauce, using as much as desired.
Cover the dish tightly with aluminum foil and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.
Remove from the oven and let it cool for a few minutes before serving. Garnish with fresh basil leaves.
Notes
Feel free to add more vegetables or spices to the filling for extra flavor.