Boil 2-3 litres of water in a pot. Wash the meat and put it into the pot (it should be covered with water). Wash and halve the onion. Add 2 teaspoons of salt, bay leaves, peppercorns and onion to the meat and bring everything to the boil.
Cover the pot to a crack and simmer the meat at low heat for 2-2 1/2 hours. Peel and wash the potatoes. Clean the kohlrabi (put some green aside for garnishing), wash and cut into slices.
Clean the carrots (leaving some of the greenery to stand) and wash them. Add the prepared ingredients to the boiled fillet of beef approx. 20 minutes before the end of the cooking time. Then take the meat out of the stock and let it rest covered for about 10 minutes.
Heat the fat in a saucepan. Sweat flour in it. Deglaze with 3/8 litres of the bouillon and milk, bring to the boil and simmer for about 5 minutes, stirring several times. Season to taste with salt, pepper and lemon juice.
Wash and peel the horseradish, grate it into coarse curls and sprinkle it immediately with some lemon juice. Arrange the boiled beef, potatoes and vegetables on a plate with some stock and sauce. Garnish with kohlrabigrün and chervil.
Horseradish and remaining sauce extra rich.