Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Dab meat dry and remove the fat layer slightly if necessary. Heat clarified butter in a flat roaster or pan.
Brown the meat in it for 10 minutes. Season well with salt and pepper.
Roast in a hot oven for 35-40 minutes. It is best to insert a roast thermometer into the thickest part of the roast beef. If it shows a core temperature of 55-65 °C, the meat is pink inside. Remove the meat and let it cool down.
Cover and place in the refrigerator for 3-4 hours.
In the meantime, wash the potatoes thoroughly, peel them if necessary and cut them into thin slices. Heat 2 tablespoons of oil in a large frying pan. Fry the potatoes for about 20 minutes while turning. Season with salt and pepper.
Pour into a large bowl.
Cut the cucumbers into thin strips. Peel and chop onions. Heat 3 tablespoons of oil. Sauté onions in it. Deglaze with vinegar, 350 ml water and cucumber water. Bring to the boil and simmer for 2-3 minutes. Stir in mustard and stock.
Season with approx. 1⁄2 TL salt and pepper and 1-2 TL sugar. Mix hot marinade, potatoes and cucumber. Leave to stand for about 30 minutes.
For the tartar sauce, wash parsley, shake dry and chop finely. Mix with sour cream and mayonnaise. Season with salt, pepper and 1 pinch of sugar. Cut the roast beef into thin slices.
Season the potato salad to taste again. Arrange everything.