Bring a large pot of salted water to a boil and cook the jumbo pasta shells according to the package directions until they are al dente. Once cooked, drain them well and set aside to cool for a few minutes.
In a large mixing bowl, combine the chopped spinach, ricotta cheese, 1/2 cup of the shredded mozzarella cheese, grated Parmesan cheese, beaten egg, garlic powder, onion powder, and a sprinkle of salt and pepper. Stir well until all ingredients are thoroughly combined into a creamy filling.
Take each cooled pasta shell and gently stuff it with the spinach and cheese mixture, being careful not to tear the shell.
In a glass or ceramic baking dish, pour 1 cup of marinara sauce evenly across the bottom to prevent sticking.
Arrange the stuffed shells in the dish with the openings facing up. Cover the shells with the remaining marinara sauce, ensuring each shell is generously coated for optimal flavor and moisture.
Sprinkle the remaining 1/2 cup of shredded mozzarella cheese evenly over the top of the stuffed shells.
Cover the dish tightly with aluminum foil and place it in the preheated oven. Bake for 25 minutes.
After 25 minutes, carefully remove the foil and bake for an additional 10-15 minutes, or until the cheese becomes bubbly and turns a lovely golden brown.
Once done, take the dish out of the oven and let it cool for about 5 minutes. Garnish with fresh basil leaves just before serving for an aromatic touch.
Notes
For a delightful presentation, serve the stuffed shells on a warm plate with extra marinara sauce and basil leaves.