01 - Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
02 - Boil jumbo pasta shells in generously salted water following package instructions until al dente. Drain thoroughly and set shells aside to cool slightly.
03 - In a medium mixing bowl, combine ricotta cheese, canned pumpkin puree, grated Parmesan, large egg, ground nutmeg, salt, black pepper, and chopped sage. Mix until homogenous and creamy.
04 - Melt unsalted butter in a saucepan over medium heat. Sauté minced garlic for 1 minute until fragrant. Add heavy cream; bring to a gentle simmer and cook for 3–4 minutes. Stir in grated Parmesan, ground white pepper, and salt. Cook, stirring, until cheese has melted and sauce is smooth. Remove from heat.
05 - Spread 1/2 cup of Alfredo sauce evenly over the bottom of the prepared baking dish.
06 - Fill each cooked pasta shell with approximately 2 tablespoons of the pumpkin-ricotta mixture. Arrange shells in a single layer in the baking dish.
07 - Pour remaining Alfredo sauce evenly over the filled shells. Sprinkle shredded mozzarella cheese over the top.
08 - Cover the dish with foil and bake for 25 minutes. Remove foil and continue baking for an additional 10–15 minutes, or until the sauce is bubbling and the top is golden brown.
09 - Garnish with chopped parsley, if desired, and serve immediately.