“Herbes” in France does not mean only herbs, as we understand the word, but also includes sorrel, lettuce, etc.
Pick and wash the sorrel and the lettuce and cut in thin strips. Pick the chervil from the stalks and chop coarsely. Put all in a saucepan with a little butter and simmer very slowly for 30 minutes, till the vegetables are quite tender. Now add the stock, cover the saucepan and simmer for another ½ an hour. Ten minutes before serving, mix the yolks of eggs in a basin with a little cold stock. Remove the saucepan from the fire, and add gradually a few spoonfuls of the warm stock to the eggs, then add the egg mixture gradually to the soup, stirring well. Replace the saucepan on the fire. Put in a small pat of butter just before serving. Small dice of bread—croûtons—fried in butter, are served with this.