Bring a large pot of salted water to a boil and gently add the jumbo shells. Cook for 8 to 10 minutes, stirring occasionally until they reach an al dente texture. Carefully drain the shells and cool them under running water.

In a medium saucepan, melt 2 tablespoons of butter over low heat. Add minced garlic and sauté for about 2 to 3 minutes until fragrant. Stir in a pinch of nutmeg and cook for another minute. Gradually whisk in the pumpkin puree and half-and-half, followed by grated Parmigiano and minced rosemary, until the sauce is smooth.

In a large mixing bowl, combine the whole milk ricotta, mozzarella, remaining Parmigiano, minced rosemary, nutmeg, salt, and pepper. Stir vigorously until the mixture is creamy and well blended.

Carefully spoon the cheese filling into each cooked jumbo shell, packing it in without overfilling. Place each shell seam side up directly into the warm pumpkin cream sauce in the saucepan.

Preheat your oven to 400°F (200°C). If desired, sprinkle additional mozzarella on top of the stuffed shells. Bake uncovered for about 20 minutes or until the sauce is bubbly and the tops are golden.

Remove the dish from the oven and let it cool for a few minutes to allow the pumpkin cream sauce to thicken slightly.