For the potato-cucumber-mint salad, peel the onion and cut into small cubes. Bring to the boil once with the vinegar, mustard and water, season with salt and a little pepper.
Boil the potatoes in enough (about 1-1.5L) water, with a tablespoon of salt until soft. Drain the water and rinse briefly with cold water.
Peel the potatoes and cut them into thin slices, pour the onion spice water over them and stir gently until the potatoes have absorbed all the marinade.
Peel the cucumber and thinly slice or chop it, together with the mint leaves and olive oil, stir into the salad. Season the potato-cucumber-mint salad again with salt and pepper.