Dab meat dry, if necessary cut off the fat layer. Rub the meat with salt and pepper. Brown all around in 3 tbsp. hot oil and place on an oiled fat pan. Bake in a preheated oven (electric cooker: 150 °C/circulating air: 125 °C/gas: level 1) for approx. 1 3⁄4 hours.
Wash the rosemary. Peel and slice the onions. Peel and wash potatoes and cut them into large pieces. Mix with 3 sprigs of rosemary and onions, season with salt and pepper. After about 45 minutes, spread around the meat on the fat pan and drizzle with 4-5 tablespoons of oil.
Keep frying, turning from time to time.
Finely crumble the bread. Pluck the needles from the rest of the rosemary. Heat the butter. Fry the bread and rosemary needles in it. Mix with black pepper. Spread on the roast beef about 20 minutes before the end of the frying time.
If necessary, pour on some water, finish frying (if you use a roast thermometer, the core temperature should be 58-60 °C).
Mix horseradish, sour cream and mayonnaise. Season with salt, pepper and 1 pinch of sugar. Remove the roast beef and let it rest for 5-10 minutes, then cut into thin slices. Serve with horseradish cream and roast potatoes.
A mixed salad tastes good with it.