In a large saucepan, saute the mushrooms, onion and parsley in butter until tender. Stir in flour until blended; gradually add broth. Bring to a boil; cook and stir for 2 minutes or until thickened. Cool slightly.
Transfer to a blender; cover and process until pureed. Return to pan. Stir in sour cream; heat through, stirring occasionally (do not boil).