Spanish Onion Soup

Peel three large Spanish onions, cut and separate them into rings, and fry in a
little butter until they are of a light brown color and quite tender. Remove and
drain on a fine sieve, and put them into a saucepan with two quarts of water. Put
the pan on the fire, boil for an hour, stirring frequently, add salt and pepper to taste,
add the finely sifted crumb of a roll, and mix thoroughly. Boil for one hour longer,
and just before serving add the yolks of two eggs, beaten into two tablespoonfuls of
vinegar and a small quantity of the soup. Mix this in, stirring one way, pour into a
tureen and serve. Soup prepared in this way will keep four or five days.