Either slice the olives wafer-thin or chop them into bits in a food processor. Place chicken stock in a medium saucepan and bring to a boil. Add onions and garlic. Simmer 10 minutes. Remove pan from heat, strain out onion and garlic bits and discard (or save to add the pot when making a batch of stock). Return soup to the pan and add olives. Beat egg in a bowl and slowly add a cup of hot soup while continuing to beat, and taking care not to curdle the egg. Pour soup/egg mixture into the soup. Add light cream, steak sauce and sherry. Refrigerate for several hours or overnight. Serve chilled in chilled soup bowls.