Preheat the oven to 375°F (190°C).
Cook the pasta shells according to package instructions until al dente.
Sauté the spinach in olive oil until wilted, about 3-4 minutes.
In a bowl, combine ricotta, mozzarella, Parmesan, sautéed spinach, garlic powder, and season with salt and pepper.
Spread a layer of marinara sauce on the bottom of a baking dish.
Stuff each pasta shell with the spinach and cheese mixture.
Place the stuffed shells in the baking dish, seam side up, and cover with remaining marinara sauce.
Cover the baking dish with foil and bake for 20 minutes.
Remove the foil and bake an additional 10-15 minutes until the cheese is bubbly and golden.
Garnish with fresh basil before serving.