Step 1: Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish.
Step 2: Cook the jumbo pasta shells in boiling salted water according to package directions, but remove them 2 minutes early so they remain firm (al dente). Drain and set aside.
Step 3: In a small skillet, sauté the garlic and chopped spinach with a touch of olive oil until wilted. Let cool slightly.
Step 4: In a large bowl, combine the pumpkin puree, ricotta, sautéed spinach, half of the Parmesan cheese, the egg, nutmeg, sage, salt, and pepper. Mix until smooth.
Step 5: Spread a thin layer of sauce on the bottom of the prepared baking dish.
Step 6: Fill each cooked shell with approximately 2 tablespoons of the pumpkin and spinach mixture and place them in the dish.
Step 7: Top with the remaining sauce, mozzarella, and the rest of the Parmesan cheese.
Step 8: Cover with foil and bake for 20 minutes, then remove foil and bake for another 10-15 minutes until the cheese is bubbly and golden.