Stuffed Shells With Creamy Butternut Sauce

Preheat oven to 425 degrees. Cut squash in half, scrape out seeds and place skin-side up on a greased or parchment-lined baking sheet. Roast for 30 to 40 minutes or until tender when pierced with a knife. Boil water for shells, and cook shells according to directions. Drain, coat with olive oil and set aside.
Meanwhile, brown sausage in a heavy-bottomed skillet over medium heat. Remove sausage from pan and set aside on a paper-lined plate. Wipe out skillet, heat olive oil and saute onions over medium heat until golden and tender. Add garlic and mushrooms. Saute on medium-low heat until mushrooms are tender. Add sage, saute for 1 minute and turn off heat. Place sausage back in pan and stir in pine nuts, ricotta and mozzarella. Add pepper and half of the salt. Stir and set aside.
When squash is tender, remove from oven, lower heat to 350 degrees, and turn squash over to cool. Spoon out 2 cups of roasted squash into a blender with cold chicken stock, milk, parmesan, remaining salt, plus sugar, nutmeg and pepper. Blend until smooth.
In a large greased baking dish or individual ramekins, pour enough sauce to generously coat the bottom. Stuff the shells with meat mixture and place them in the baking dish or ramekins over the sauce. Pour more sauce over top. Cover with foil and bake until heated throughout, about 20 minutes. Remove foil and continue baking. Add additional cheese, if desired, or any remaining sauce to the top, and bake for 10 more minutes.