Preheat oven to 350 degrees.
Cut squash in half lengthwise and scoop out the seeds. Arrange the halves in roasting pan that has been greased with olive oil or butter. Bake squash in the oven for 40-45 minutes or until very tender. Set aside to cool.
When the squash is completely cool, scoop the flesh from the skin.
While the squash is baking, cook the onion and the celery (optional ginger) in the butter in a saucepan, over moderately low heat, for 5 minutes until the onion and celery are softened.
Add the broth and simmer the mixture for 10 minutes, covered, add the squash pulp to the saucepan.
Transfer the mixture to a blender or food processor, in batches, and puree until smooth. Add enough water to achieve the desired consistency, and salt and pepper to taste.
Return the soup to the saucepan, add cream and cook over moderate heat until hot.
Chef's Notes: Garnish each serving with a heaping teaspoon of real sour cream