Wash the meat, dab dry, remove all but a small amount of fat. Heat 2 tablespoons of oil in a flat roasting pan or an ovenproof pan. Brown the meat in it for 6-7 minutes, season with salt and pepper.
Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 35-40 minutes. It is best to use a meat thermometer and cook up to 58 °C core temperature. (Or cook in a preheated oven (electric cooker: 100 °C/ convection oven: 75 °C/ gas: unsuitable) for approx. 1 1/2 hours, also with a meat thermometer).
In the meantime, wash the potatoes thoroughly and cut them into thin slices. Heat 1 tablespoon of oil in a large, coated pan. Fry the potatoes for 5-10 minutes while turning. In the meantime, cut the cucumbers into strips.
Onions peel and in larger cubes cut. Bring onions, stock, vinegar, 4 tablespoons of oil and mustard to the boil, season with 1 level teaspoon of salt, 1/2 teaspoon of pepper and 1 teaspoon of sugar. Put the cucumbers, potatoes and marinade in a bowl, mix and leave to stand for about 15 minutes.
Remove the meat, wrap it in aluminium foil and let it rest in a warm place for 5-10 minutes. Wash parsley, dab dry, pluck leaves from the stalks and chop finely, except for something to garnish.
Mix parsley, sour cream and mayonaise, season with salt and pepper. Season salad again. Remove meat, cut into slices, arrange with salad and dip. Garnish with parsley.