In a wide frying pan over medium-high heat, cook mushrooms, onion, and parsley in butter until mushrooms are soft and pan juices have evaporated (about 5 minutes).
Stir in flour and cook for 1 minute to brown lightly.
Slowly stir in broth; bring to a boil, stirring.
In a blender or food processor, puree soup with sour cream, a portion at a time, until smooth.
Reheat without boiling.