Wash the chicken breast and put it in a pot with 1 1/2 litres of cold water so that the chicken breast is covered. Bring to the boil. Peel 1 onion, carrot and celeriac. Clean and wash the leek. Cut carrot, onion, leek and celery into pieces and add to the chicken breast.
Wash the remaining onion and cut in half. Heat the oil in a pan and fry the onion on the cut side until it is golden brown. Add to the chicken breast. Simmer the soup for about 1 1/4 hours. In between, skim off the floating foam with a skimmer. Remove the chicken breast. Pass the broth through a sieve lined with kitchen paper. Cut bread into cubes. Heat butter in a pan, add chilli flakes and sesame seeds. Fry the bread cubes until crisp and finally add herbs.
Remove the chicken breast. Pass the broth through a sieve lined with kitchen paper. Cut bread into cubes. Heat butter in a pan, add chilli flakes and sesame seeds. Fry the bread cubes until crisp and finally add herbs. Heat up about 1 litre of stock, season again. Put the croutons in deep plates, pour in broth and serve hot