Cook the pasta shells according to the package instructions until al dente. Drain and set aside.
In a mixing bowl, combine ricotta cheese, mozzarella cheese, Parmesan cheese, chopped spinach, basil pesto, garlic powder, salt, and pepper. Mix well until the ingredients are fully incorporated.
Spoon a generous amount of the cheese mixture into each cooked pasta shell and place them seam side up in a baking dish.
Spread the marinara sauce evenly over the stuffed shells, covering them completely.
Sprinkle any remaining mozzarella and Parmesan cheese on top of the marinara sauce for extra cheesiness.
Cover the baking dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and slightly golden.
Once done, let it cool for a few minutes, then garnish with fresh basil leaves before serving.
Notes
Feel free to add more vegetables or use different types of cheese.