Cut squash in half lengthwise, and scoop out the seeds. Arrange the halves cut side down in roasting pan that has been sprayed with nonstick vegetable oil spray. 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 ginger in the butter in a saucepan, over moderately low heat, for 5 minutes or until the onion is softened, Add the broth and simmer the mixture for 10 minutes, covered. Add the squash pulp to the sauce pan. 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 sauce pan and cook over moderate heat until it is hot. Garnish each portion with the heaping teaspoon of low-fat sour cream.