Begin by bringing a large pot of salted water to a rolling boil. Gently add the jumbo shells and cook for 8-10 minutes until al dente. Drain and cool slightly.

In a sauté pan, melt 4 tablespoons of butter over medium heat. Add minced garlic and cook until fragrant, about 1-2 minutes. Stir in nutmeg, then whisk in pumpkin puree and half and half. Blend until smooth.

In a mixing bowl, combine ricotta, mozzarella, and Parmigiano Reggiano. Add in minced rosemary, nutmeg, and season with salt and pepper. Mix until creamy.

Carefully stuff each shell with the cheese mixture. Arrange filled shells seam side up in a baking dish, nestled in the pumpkin sauce.

Preheat the oven to 400°F (200°C). Cover the baking dish with aluminum foil and bake for 20 minutes. Remove foil halfway through for a golden top.

Let cool for 5-10 minutes before serving. Serve warm, garnished with fresh herbs if desired.