Cook the jumbo pasta shells according to the package instructions until al dente. Drain and set aside to cool slightly.
In a large mixing bowl, combine the ricotta cheese, mozzarella cheese, Parmesan cheese, chopped spinach, beaten egg, garlic powder, onion powder, Italian seasoning, salt, and pepper. Mix well until all ingredients are evenly incorporated.
Spread 1 cup of marinara sauce evenly on the bottom of a 9x13 inch baking dish.
Fill each cooked pasta shell with about 2 tablespoons of the cheese and spinach mixture and arrange them in the baking dish on top of the marinara sauce.
Once all shells are filled and arranged, pour the remaining marinara sauce over the stuffed shells, ensuring they are all covered.
Sprinkle a generous amount of shredded mozzarella cheese over the top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let it sit for 5 minutes before serving. Garnish with fresh basil leaves.
Notes
Let the dish sit for a few minutes before serving for better slicing.