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 workbowl 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.