In a 7x11 inch baking dish, heat the butter for 30 seconds in the microwave to melt, Add the Mushrooms, onions, and parsley and microwave for 5 minutes, stirring once or twice. Take dish out stir in the flourand 1/2 cup of the beef broth. cook uncovered, for 1 or 2 minutes or untill it's bubbly. In a Blender,- working in Batches - Puree the mixture add in sour cream and rest of beef broth, return the soup back to the baking dishand re- heat for 3 minutes.
If you make the soup ahead of time, cover and refrigerate. To reheat the soup, cover and place in microwave for 3 minutes. Ladle the soup into Bowls and floate the mushroom slices that were saved on top.