Bring a large pot of salted water to a boil. Add the 20 jumbo pasta shells and cook until al dente, about 8 to 10 minutes. Drain in a colander and rinse with cold water. Set aside to cool slightly.
In a mixing bowl, combine 2 cups ricotta cheese, 2 cups chopped fresh spinach, 1 egg, 1/2 cup grated Parmesan cheese, 1 teaspoon garlic powder, 1 teaspoon onion powder, salt, and pepper. Stir until well mixed.
Using a spoon or small spatula, gently fill each cooked shell with the ricotta-spinach mixture. Place stuffed shells seam-side up in a greased 9x13 inch baking dish.
Pour 2 cups marinara sauce evenly over the stuffed shells. Sprinkle 1 cup shredded mozzarella cheese on top.
Preheat oven to 375°F (190°C). Cover the baking dish with foil and bake for 25 minutes. Remove foil and bake an additional 10 minutes until cheese is melted and bubbly with golden spots.
Remove from oven and let rest for 5 minutes. Garnish with fresh basil. Serve hot and enjoy.