How to Make It
Preheat the oven to 375°. In a large roasting pan, toss the squash with the 4 tablespoons melted butter and a pinch each of salt and pepper. Roast for about 40 minutes, stirring occasionally, until the squash is tender.
Meanwhile, melt the remaining 4 tablespoons butter in a large saucepan. Add the onion, celery, leek and carrot and cook over moderate heat, stirring occasionally, until barely softened, about 8 minutes. Stir in the ginger, thyme and roasted squash. Add the stock and bring to a boil over high heat. Partially cover, reduce the heat to moderately low and simmer until the vegetables are very tender, about 20 minutes.
Working in batches, puree the soup in a blender. Return the soup to the saucepan and season with salt and pepper.
Reheat the soup. In a medium bowl, whip the heavy cream until soft peaks form. Add the scallions and a pinch of salt and whip to firm peaks.
Ladle the soup into bowls or shallow soup plates and garnish with a dollop of the scallion cream and a sprinkling of the chives. Serve at once.