Onions peel and cut into slices. Peel, wash and cut carrots into large pieces. Clean, wash and cut the leek into large pieces. Wash marjoram and shake dry
Wash the meat, dab dry and rub all around with salt and pepper. Heat the oil in a large frying pan, fry the meat thoroughly all around, remove. Fry the onions, leek and carrots in the frying fat while turning them over.
Insert a meat thermometer into the core of the meat. Place meat in the roaster, add marjoram. Pour the stock, bring to the boil. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 1 1/4 hours (core temperature 67 °C)
In the meantime, peel the potatoes, wash them thoroughly and cook them in boiling salted water for about 25 minutes. Peel, wash and cut the kohlrabi into slices. Wash and quarter apples, cut out the core. Cut quarters into slices and sprinkle with lemon juice
Remove the roast from the oven, remove from the stock, keep warm. Pour stock through a sieve into a pot. Skim off fat. Add vinegar, bring to the boil and simmer for 12-15 minutes. Season to taste with salt and pepper
In the meantime heat 25 g butter in a large, wide pot. Add the kohlrabi and apple and fry briefly while turning. Sprinkle with sugar and let it caramelise briefly. Deglaze with stock, bring to the boil, cover and stew for about 10 minutes
Heat milk and 75 g butter. Drain potatoes, add hot milk and horseradish, mash with a potato masher. Season to taste with salt and nutmeg
Wash parsley, shake dry and cut into strips. Mix parsley into the vegetables. Cut the meat open, arrange on a plate with the vegetables and stock garnished with parsley. Add the mashed potatoes and horseradish in a bowl