Peel onion and carrots, chop coarsely. Wash the chicken inside and outside. Put onion, carrots and chicken in a high pot, pour on water until the chicken is covered with water (about 3 litres).
Add bay leaf and peppercorns, season with salt. Slowly bring to the boil and simmer for about 1 1/2 hours.
Clean the leek, cut it in half lengthwise, wash it and cut it into fine strips. Heat butter and oil in a frying pan and sauté the leek in it. Take the chicken out of the pot and let it cool down a little. Pour broth through a sieve.
Measure out 2 litres of stock and bring to the boil in a pot. Remove flesh from skin and bones and chop. Add leek and meat to the stock and simmer for about 5 minutes. Season soup again.