Peel 2 onions and cut into large pieces. Peel and wash 2 carrots and also cut them into large pieces. Fry them briefly in a large pot without fat. Add approx. 2 l water, peppercorns and 1 tsp salt and bring to the boil.
Wash and add the boiled beef, cover and simmer for approx. 1 1⁄2 hours. Skim off the foam from time to time.
Clean or peel and wash the remaining vegetables. Cut the leek into thin rings. Cut carrots into thin slices. Peel the remaining onions and cut into rings. Peel and wash the potatoes and cut them into small cubes.
Heat 2 tablespoons of butter in a large frying pan. Stir-fry the potatoes and onions for 12-15 minutes until crispy. Season with salt and pepper.
Lift out the boiled fillet of beef. Remove approx. 200 ml of the meat stock and sieve. Cook the carrots and leek for 8-10 minutes. Peel and coarsely grate the horseradish. Mix immediately with lemon juice.
Cut the boiled fillet of beef into approx. 1 1⁄2 cm thick slices. Remove the potatoes. Heat 2 tablespoons of butter in the frying fat. Fry meat slices in it in 2-3 portions on each side for 1-2 minutes. Serve, sprinkle with horseradish.