Set aside several sprigs of sorrel for the garnish. Cut the stems away from the leaves and discard. Cut the sorrel roughly. Melt the butter in a large saucepan and cook the sorrel over a moderate heat, stirring, for 5 minutes. Sprinkle with flour and cook for 1-2 minutes. Add 8 cups of water and bring to a boil. Add the potatoes and cover. Reduce heat and simmer for 20 minutes. PurÄe the soup in a food processor or blender. Return the soup to the casserole. Chop the reserved sorrel sprigs into julienne and stir them into the soup. Reheat the soup gently. Just before the soup boils, stir in the crÅme fraöche. Season with salt but do not allow the soup to boil. At least 10 minutes before serving whisk together the egg yolk and chopped chervil in the bottom of a soup tureen. Stir a ladleful of the soup into the herb mixture. Slowly add the remaining soup to the tureen. Serve the soup with croutons on the side. You can substitute 1 cup of rice for the potatoes, but don╒t add the flour.
