Pour boiling water over the mushrooms and let them stand for a few minutes. Then boil them in the stock till tender, remove when done, chop and return to the stock. Melt the butter in a small saucepan, stir in the flour, without browning, moisten with a few tablespoons of the hot stock and stir into the soup. Just before serving, add the sour cream. Nouilles are sometimes served with this typically Polish soup.