Rinse barley thoroughly until water runs clear. Put barley in large stockpot with enough water to cover. Cook for approximately 25 minutes, adding water as needed. Saute onions in frying pan with no oil or butter – just until water leaves the onions. Then continue to sauté in butter, NOT margarine. Add onions to barley. Add mushrooms. Cook for another 25 minutes. Add whipping cream. Add sour cream to taste. Add beef base bouillon cubes and continue to cook, stirring continuously. Do not boil. Add white pepper. When soup is warm, serve.