Wash the sorrel thoroughly in several changes of cold water. Remove and discard the stems. Chop the leaves into thin ribbons. You can make this with spinach but the taste will be different. Sorrel has a sour/bitter flavor.
Heat the butter or oil in a 3 quart saucepan and saute, the sorrel and onions.
Cook stirring, for 10 minutes until the sorrel is wilted and the onion translucent. Add water and salt.
Let the soup simmer for 25 to 30 minutes more.
Remove from heat and stir in sugar. Add the lemon juice a little at a time, tasting constantly to achieve the degree of tartness that pleases you.
Beat a tablespoon or two of the soup into the egg yolks, then stir egg yolks into the soup. Reheat the soup but do not let it boil after adding the egg yolks.