For the chervil soup, dice the onion very finely. Melt 25 g butter in a saucepan and sauté the onions.
Peel and dice the potato and add to the onions, pour in the soup and cook for ten minutes without a lid.
Pluck the chervil, wash and drain. Puree finely with the remaining, room-warm butter in a blender or with a hand blender. Gradually add the cream and hot soup. Blend until smooth and season to taste with salt and pepper.
Decorate the chervil soup with fresh chervil sprigs.