Cook squash, remove pulp. Combine 1/2 cup stock, apples and onion in heavy medium saucepan. Cover and cook over low heat for 10 minutes. Add squash pulp, remaining stock, applejuice, ginger and salt. Cover and simmer until ingredients are very tender,
Puree soup in batches in blender or processor. Strain through sieve into clean saucepan, pressing puree with back of spoon. Reheat soup gently. Season with salt and generous amount of pepper. Ladle into bowls. Garnish with yogurt or sour cream and chives.