Peel and slice the boiled potatoes. Cut the cucumber, bacon and onion into cubes. Add the cucumbers to the potatoes.
Fry the bacon cubes until crispy, pour off the liquid fat. Then add the onion cubes and sweat until translucent (do not let them turn brown). Then add to the potatoes.
Add the oil, vinegar and cucumber liquid to the potatoes. Mix everything well and season with salt and pepper. Leave to steep in the refrigerator for approx. 2-3 hours, but preferably overnight, until consumed. If necessary, season and add additional cucumber liquid if necessary.
Ideal as an accompaniment to a grilled steak or a roster (bratwurst).