Step 1: Preheat your oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.
Step 2: Cook the jumbo pasta shells in a large pot of boiling salted water according to package directions, but drain them 2 minutes early so they remain firm (al dente). Rinse with cold water to stop the cooking.
Step 3: In a medium skillet, sauté the minced garlic and chopped spinach until the spinach is completely wilted. Squeeze out any excess moisture using a paper towel.
Step 4: In a large mixing bowl, combine the pumpkin puree, ricotta cheese, beaten egg, sautéed spinach, half of the Parmesan cheese, sage, nutmeg, salt, and pepper. Stir until well combined.
Step 5: Spread about 1/2 cup of your chosen sauce (marinara or bechamel) onto the bottom of the prepared baking dish.
Step 6: Stuff each cooked shell with about 2 tablespoons of the pumpkin and spinach mixture. Place the shells open-side up in the baking dish.
Step 7: Top the stuffed shells with the remaining sauce, shredded mozzarella, and the rest of the Parmesan cheese.
Step 8: Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes until the cheese is bubbly and lightly browned.
Step 9: Let the shells rest for 5 minutes before serving to allow the filling to set.