Peel and chop the onion. Celery and potatoes peel, wash and roughly dice. Fry everything in hot fat while stirring. Deglaze with 3/4 l water, bring to the boil. Stir in broth and simmer covered for about 20 minutes
Wash the herbs and put some aside for garnishing. Pluck the chervil leaves and cut the chives into small rolls. Puree the soup and stir in half the cream
Take the pot off the stove. Whisk the egg yolk and the rest of the cream and stir into the soup (do not boil any more!). Season to taste. Stir in herbs. Garnish with the rest of the herbs. Goes well with toast bread