Cook and stir onions in oil in a 3-quart saucepan until tender.
Stir in remaining ingredients except broth and sour cream.
Cover and cook over medium heat for 5 minutes, stirring occasionally.
Place mixture in food processor work bowl fitted with steel blade or in a blender container; cover and process until smooth, about 1 minute.
Return mixture to saucepan; stir in broth. Heat to boiling; reduce heat.
Simmer uncovered for 10 minutes. Stir in sour cream.
Cover and refrigerate any remaining sauce.