Butternut Squash Soup

1 medium butternut squash (about 2 1/4 pounds)
Nonstick vegetable oil spray
1 medium onion, chopped (about 1 cup)
1 Tbsp. freshly grated ginger - optional
3 Tbsp. unsalted butter
3 cups chicken broth
1-2 cups water, as needed
Sour cream for garnish

Preheat oven to 350°. 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 �?until the onion is 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 and cook over moderate heat until it's hot. Garnish each portion with a heaping teaspoon of low-fat sour cream.