Place all ingredients in a blender except sour cream.
Puree until smooth.
Refrigerate until cold (you may prepare up to 2 days in advance).
Just before serving, stir in sour cream.
Serving suggestions: To keep the soup cold while serving, place a smaller bowl inside a large bowl and add ice to the larger bowl. Place the cold soup in the smaller bowl. Serve the soup in small bowls or single serving "shot glasses".