Bring a large pot of salted water to a boil. Add the 12 large pasta shells and cook according to the package instructions until al dente, usually about 8-10 minutes. Drain the shells and drizzle with 1 tablespoon olive oil to prevent sticking. Set aside to cool slightly.
In a large mixing bowl, combine the ricotta cheese, chopped spinach, shredded mozzarella, grated Parmesan, garlic powder, onion powder, salt, and black pepper. Mix until evenly incorporated.
Using a spoon or small cookie scoop, carefully fill each cooked pasta shell with the ricotta and spinach mixture. Place the stuffed shells seam side up in your prepared baking dish.
Spoon about half of the marinara sauce on the bottom of the baking dish. Arrange the stuffed shells in a single layer on top of the sauce. Pour the remaining marinara sauce evenly over the shells to cover completely.
Cover the baking dish loosely with aluminum foil and bake in a preheated oven at 375°F (190°C) for 25 minutes. Remove the foil and bake for an additional 10 minutes or until cheese is melted and sauce is bubbling. Optionally broil for 1-2 minutes for a golden top.
Let the shells rest for a few minutes. Garnish with fresh basil leaves and serve warm with a side of garlic bread or a light salad.
