Cover potatoes with skin and cook in water for about 20 minutes. Drain, rinse with cold water and peel. Let potatoes cool down for about 45 minutes.
In the meantime, finely dice the cucumbers for the tartar sauce. Wash the chives, shake dry and cut into fine rolls. Mix yoghurt, salad cream and cucumber water. Stir in the cucumber, chives and capers.
Season to taste with salt and pepper.
Peel and finely chop the onion. Cut the potatoes into slices. Mix potatoes and oil carefully. Heat a large coated pan. Fry the potatoes in it in portions over medium heat until crispy brown, turning occasionally.
Fry the onion for about 5 minutes. Season the fried potatoes with salt and pepper.
Arrange potatoes, roast beef and some tartar sauce. Add the rest of the tartar sauce.