French Onion Soup Gratinée

Procedure

1. Heat the butter in a stockpot over moderate heat. Add the onions and cook until golden. Stir occasionally.
2. Add the stock and bring to a boil. Simmer until the onions are very tender and the flavors are well blended, about 20 minutes.
3. Season to taste with salt and pepper. Add the sherry, if desired.
4. Keep the soup hot for s