How to Make It
Preheat the oven to 300°. Set the squash, cut sides up, on a rimmed baking sheet lined with foil. Add 1/2 teaspoon of the butter to each squash and season with salt and pepper. Add 1/8 inch of water to the baking sheet. Roast the squash for 45 minutes, or until tender.
In a large saucepan, melt the remaining 2 tablespoons of butter. Add the onion and thyme and cook over low heat, stirring occasionally, until the onion is softened but not browned, about 5 minutes. Scrape the flesh out of the squash and add it to the saucepan along with the stock and heavy cream. Cook over moderate heat, stirring occasionally, until the liquid has reduced by one-fourth, about 20 minutes.
Puree the soup in batches in a blender or food processor. Strain into a clean saucepan and season with salt and pepper. Ladle the soup into soup plates and garnish with a dollop of crème fraîche and a fresh truffle shaving if desired.