Tie the ribs and stems of the sorrel leaves into a secure bundle.

Bring water with sorrel leaves and bundle of ribs and stems to a boil in a nonreactive saucepan. Reduce heat to low and simmer 20-30 minutes, until leaves are soft and starting to lose texture. Discard bundle of ribs and stems.

Lightly beat eggs and egg yolk with a fork in a large bowl. Slowly beat in the hot soup. When four cups soup have been added, trickle egg mixture back into the saucepan, beating constantly. Pour soup back and forth between the pot and bowl to cool it more quickly. Let cool and refrigerate until cold.

Stir in lemon juice, and salt and pepper to taste just before serving. Serve with sour cream.