Preheat your oven to 375°F (190°C).
In a large pot of salted boiling water, cook the pasta shells for about 8 minutes until al dente. Drain and set aside to cool slightly.
In a mixing bowl, combine the ricotta, mozzarella, Parmesan, cooked spinach, garlic powder, salt, black pepper, and nutmeg. Mix until well blended.
Carefully stuff each pasta shell with the cheese mixture and place them in a greased 9x13-inch baking dish.
In a saucepan over medium heat, combine the pumpkin puree, heavy cream, olive oil, sage, and cinnamon. Stir continuously until the mixture is heated through and smooth, about 5 minutes.
Pour the pumpkin cream sauce evenly over the stuffed shells, ensuring they are well coated.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and slightly golden.
Let the dish cool for a few minutes before garnishing with fresh basil. Serve warm and enjoy!