Boil a large pot of salted water and cook the jumbo pasta shells according to package instructions until al dente. Drain and rinse with cold water to prevent sticking.
In a mixing bowl, combine the ricotta cheese, chopped spinach, grated Parmesan cheese, shredded mozzarella cheese, egg, garlic powder, Italian seasoning, salt, and pepper. Mix well until all ingredients are incorporated.
Take each cooked pasta shell and fill it with about 2 tablespoons of the ricotta-spinach mixture. Place the stuffed shells seam side up in a greased baking dish.
Spread half of the marinara sauce over the stuffed shells evenly. Add extra marinara to ensure the shells are covered.
Cover the baking dish with foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10-15 minutes until the cheese is melted and bubbly.
Once baked, remove from the oven and let cool for a few minutes. Garnish with fresh basil leaves before serving.