Step 1Peel squash and cut into quarters lengthwise. Remove seedy portion. Cut squash into cubes. Place squash and water in saucepan and bring to boil. Reduce heat, cover and simmer until squash becomes translucent and tender, about 10 minutes. Set aside a few cubes of squash for garnish.
Step 2Heat butter in small skillet over medium-low heat until melted. Add garlic and onion and saute until tender, about 2 minutes. Add to cooked squash in saucepan. Stir in broth. Raise heat and simmer about 1 minute.
Step 3Puree squash mixture in blender until smooth. Stir in vinegar and salt. Chill 1 hour or until ready to serve. Sprinkle a few reserved squash cubes on top of soup and garnish with dollops of sour cream.