Melt the butter in a saucepan and sweat the onions and potatoes in it. Cover and steam over low heat for about 20 minutes. Deglaze with the chicken stock, bring to the boil and add the lettuce and chervil. Let it boil for a few minutes and pass through a sieve (or Flotte Lotte). Heat the soup again and stir in the cream. Season with salt and pepper and sprinkle with chopped chives and chervil.