Preheat oven to 425 degrees. Cut squash in half and scoop out the seeds. Place butternut squash halves on a large baking sheets and brush with 1-2 tablespoons olive oil. Season with salt and pepper, to taste. Bake for about one hour, or until soft. Let cool to room temperature, then scoop cooked squash into a food processor or blender.

To the squash, add 1/4 cup heavy cream, 1 tablespoon butter, 1 clove garlic, fresh thyme, nutmeg and 1/4 cup Parmesan cheese and process until smooth. Add more cream as needed until squash puree is the consistency of thick mashed potatoes. Add squash puree to a large bowl and set aside.

Preheat oven to 350 degrees. Cook jumbo shells according to package directions. Rinse with cool water and drain. Toss with a tablespoon of olive oil; set aside.

To the butternut squash puree, add the ricotta and mozzarella cheese, egg, sour cream, and salt and pepper, to taste.

To make teh sauce, add the butter and garlic to a medium saucepan and cook until garlic is fragrant, 2-3 minutes. Pour in the cream and bring to a simmer, stirring often. Add the 1/2 cup Parmesan cheese and cook a few minutes more, until cheese is melted and sauce has thickened slightly. Season with salt and pepper, to taste

Spread a thin layer of the sauce in the bottom of a lightly greased 9x13 inch baking dish. Fill the cooked pasta shells with the butternut/ricotta mixture and arrange in a single layer in the pan. Pour the remaining sauce over the shells.

Cover and bake for 20-25 minutes. Uncover and bake for about 10 more minutes, or until sauce is hot and bubbly. Let stand 5 minutes before serving.