Wash the chicken breast and put it in a pot with 2 litres of cold water. Bring to the boil. Peel 1 onion, carrots and celeriac. Clean and wash the leek. Cut 1 carrot, onion and half of the leek and celery into walnut-sized pieces and add to the chicken breast.
Wash the rest of the onion and cut in half. Heat oil in a pan and fry the onion on the cut side until golden brown and add to the chicken breast. Let the soup simmer for about 1 1/4 hours.
Cut the remaining carrot, leek and celery into fine strips. Remove the chicken breast. Line a sieve with kitchen paper, pour broth through the sieve and collect in a pot. Remove meat from skin and bone and cut into small cubes.
Cook the julienne vegetables for about 3 minutes in the simmering broth. Season to taste with salt. Add meat cubes and heat up in the broth. Wash parsley, cut leaves into strips and sprinkle over the soup.
Arrange the soup in soup plates.