If possible, cut the jacket potatoes, boiled the day before, into cubes and place in a bowl.
Finely dice the onions and cucumber and mix with the remaining ingredients. Carefully fold everything under the potatoes. If necessary, season with salt and pepper. Depending on the degree of spiciness you want, you can also choose the quality of the mustard.
Put the salad in a cool place and let it steep for 2-3 hours. Better still, overnight!