Peel and wash the potatoes and vegetables. Quarter potatoes. Cut carrots diagonally into slices, kohlrabi into small sticks. Clean and wash spring onions and cut them diagonally into rings. Cook potatoes in boiling salted water for 15-20 minutes. After about 5 minutes add carrots and kohlrabi. Add the spring onions about 3 minutes before the end of the cooking time
Wash the meat and dab dry. Heat the oil in a large pan. Fry the meat for 8-10 minutes while turning, season with salt and pepper. Remove from the pan, wrap in foil and leave to rest for about 10 minutes. Pour vegetables and potatoes into a sieve and drain
Wash the chervil, shake dry and pluck the leaves from the stems. Bring the stock and lemon peel to the boil, stir the starch with lemon juice until smooth, add to the stock and bring to the boil for about 3 minutes, stirring continuously. Remove from the hot plate and stir in the sour cream, season with salt and pepper. Arrange vegetables, sauce and 2 medallions each on plates and garnish with chervil