Wash the potatoes and wrap them in aluminium foil. Bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 1 hour. In the meantime hard-boil eggs for about 10 minutes. Then peel. Chop 1 egg, cut remaining eggs into slices. In the meantime, drain the tuna and cut into small pieces.
Wash the cucumber and cut into thin slices. Mix mayonnaise, yoghurt, half the tuna and capers. Season with salt, pepper and cucumber water. Carefully fold in the egg slices. Take potatoes out of the oven, wrap them in foil and arrange with cucumber slices, tuna mayonnaise, remaining tuna and chopped egg. Serve garnished with parsley and caper apples