Peel and finely chop the onion. Peel, wash and cut the potatoes into small pieces. Melt the fat in a saucepan. Fry potatoes and onion in it. Add stock and vinegar.
Bring to the boil, cover and cook for about 20 minutes. Meanwhile wash the meat, dab dry and cut into small pieces. Heat oil in a coated pan. Fry the meat for 4-5 minutes, season with salt and pepper.
Wash the chervil, dab dry, remove coarse stalks and put some leaves aside for garnishing. Coarsely chop remaining chervil leaves. Finely puree the soup, stir in the cream and season to taste with salt, pepper and sugar.
Add chicken pieces and chervil to the soup and bring to the boil again. Serve the soup, sprinkle with the remaining chervil and pink berries.