In a soup pot, bring the water, potatoes, dill & scallions to a medium boil & cook for 5 minutes. Add the chopped sorrel & lemon juice, salt, pepper & sugar. Simmer for 10 minutes. Remove from heat & discard the dill sprigs.
In a large bowl, beat the eggs with the cold water till ight. Very slowly pour 2 cups of the hot stock into the egg mixture, stirring constantly to prevent curdling. Then pour egg mixture back into the soup pot, stirring thoroughly. Chill until very cold, at least 3 hours.
Just before serving, whisk in the sour cream & garnish with dill. Top with cucumbers.
"Sundays at Moosewood Restaurant"