Mix all ingredients except sour cream in 3 1/2- to 6-quart slow cooker.
Cover and cook on low heat setting 8 to 10 hours (or high heat setting 3 to 5 hours) or until squash is tender.
Place one-third to one-half of the mixture at a time in blender or food processor. Cover and blend on high speed until smooth. Return mixture to cooker.
Stir in sour cream. Cover and cook on low heat setting 15 minutes or just until soup is hot; stir.
Garnish each serving with a dollop of sour cream.