MELT BUTTER IN A 2 litre POT over medium heat on the stove. Add the onions and cook covered, stirring occasionally, for 10 minutes or until the onions are soft. Add the water and the ribs, increase heat to high and bring to a boil. Boil, skimming the soup for 5 minutes. Reduce the heat to low, cover and cook for 1 hour. Add the barley and cook 20 minutes. Add the sherry, thyme, salt and pepper and mushrooms. Simmer an additional 15 minutes.