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 chopped spinach, ricotta cheese, 1 cup of mozzarella cheese, Parmesan cheese, beaten egg, garlic powder, Italian seasoning, salt, and pepper. Mix well until all ingredients are well incorporated.
Using a spoon, carefully fill each cooked pasta shell with the spinach and cheese mixture.
Spread 1 cup of marinara sauce evenly across the bottom of a 9x13 inch baking dish.
Place the stuffed shells seam side up in the baking dish. Pour the remaining marinara sauce over the top of the shells.
Sprinkle the remaining mozzarella cheese on top of the sauce.
Cover the dish with foil (to prevent sticking, you can spray the foil with cooking spray) and bake for 25 minutes.
Remove the foil and bake for another 15 minutes or until the cheese is bubbly and golden.
Once out of the oven, let it sit for a few minutes before serving. Garnish with fresh basil leaves for a pop of color.
Notes
For extra flavor, consider adding red pepper flakes to the filling.