Chop fresh mushrooms. Melt butter into a casserole. Lightly brown chopped mushrooms and onion in melted butter. Stir in flour, then warm chicken broth. Simmer for approximately 5 minutes. Puree into a blender. Pour mixture back into casserole. Stir in milk and chopped parsley. Simmer for 4 minutes. Serve, decorated with whole fresh mushrooms.