Peel, wash and chop the potatoes. Clean, peel, wash and dice carrots and kohlrabi. Wash the fillets and dab them dry. Bring the stock to the boil. Put fillets in and let them simmer at low heat for about 20 minutes.
Cook the potatoes in boiling salted water for about 15 minutes. Add carrots and kohlrabi to the potatoes about 5 minutes before the end of the cooking time. Wash the herbs, dab dry and remove leaves, except some for garnishing, from the stems and chop finely.
Melt the fat in a pot. Dust flour on it and sweat it. Take 1/2 litre of the meat-vegetable broth and pour it into the roux while stirring. Bring to the boil and simmer for 5 minutes. Stir in cream and 2/3 of the herbs.
Season to taste with salt. Drain vegetables. Remove meat from the stock, turn in the remaining herbs and cut into slices. Arrange vegetables, meat and sauce. Garnish with herbs.