In a large pot, bring salted water to a boil. Cook the jumbo pasta shells according to package instructions until al dente. Drain and rinse under cold water.
In a mixing bowl, combine ricotta cheese, chopped spinach, grated Parmesan, shredded mozzarella, egg, garlic powder, Italian seasoning, salt, and pepper. Mix until smooth.
Stuff each cooked pasta shell with about 2 tablespoons of the ricotta-spinach mixture and place seam side-up in a greased baking dish.
Pour half of the marinara sauce over the stuffed shells, ensuring they are covered.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 to 15 minutes until the cheese is melted and bubbly.
Let cool for a few minutes before garnishing with fresh basil leaves and serving.
Notes
You can use homemade or store-bought marinara sauce for convenience.