Preheat oven (electric cooker: 125 °C/circulating air: 100 °C/gas: see manufacturer). Wash meat, dab dry. Rub with salt. Heat oil in a roaster. Brown the meat thoroughly all around. Season with pepper and roast in a hot oven 1 1⁄4-1 1⁄2 for several hours (it is best to measure the core temperature with a roast thermometer, it should be 58-60 °C).
Meanwhile, cover the potatoes with their skins and cook for about 20 minutes. Add eggs and cook for the last 8-10 minutes. Drain. Quench eggs and potatoes cold, let them cool down a bit and peel them.
Let it cool down.
Finely dice the cucumbers for the tartar sauce. Finely chop the capers. Wash parsley and chives, shake dry and chop finely. Mix mayonnaise, 1 tsp. mustard and cucumber stock. Cut eggs into fine cubes and stir in cucumbers, capers and half of the herbs.
Season to taste with salt and pepper.
Wash the lemon hot, dry and finely grate the peel. Peel garlic and chop finely. Mix with lemon peel and other herbs. Season with salt and pepper.
Peel and finely dice the onions. Heat clarified butter in 2 portions in a large coated pan. Fry the potatoes in 2 portions for 8-10 minutes, turning them over. After half of the frying time add half of the onions.
Keep ready roasted potatoes warm in the oven.
Remove the roast beef from the oven, spread 1 tablespoon of mustard on the top and sides. Spread the lemon and herb mixture on top. Cut the roast beef into slices and serve with fried potatoes and tartar sauce.