Wash lemon hot and cut into slices. Put 1 lemon slice aside for garnishing. Bring 3/8 litres of slightly salted water to the boil. Wash the meat. Add lemon slices, meat and peppercorns to the salted water and cook covered over a low heat for about 15 minutes.
In the meantime, peel the carrots, wash them and cut them lengthwise into thin slices with a peeler. Clean and wash spring onions and cut them into pieces about 4-5 cm long. Steam vegetables in a little salted water for 5 minutes.
Remove the chicken filet from the stock and keep warm. Pour the stock through a sieve. Measure 1/8 liter and boil up again. Mix yoghurt and starch and thicken the stock. Wash the chervil, dab dry and put aside a little to garnish.
Chop the rest coarsely and stir into the lemon-yoghurt sauce. Season to taste with salt and pepper. Cut the chicken filet into thin slices. Arrange vegetables, meat and sauce on a plate. Garnish with lemon and chervil.
Add a slice of toast.