Wash the potatoes, cook them in salted water, drain, peel and cut into slices. Peel and chop the onion, mix with mayonnaise and half of the cucumber liquid. Season to taste with mustard, salt and pepper.
Wash and clean tomatoes and chives, quarter and core tomatoes, cut chives into rolls. Add both with the cucumber slices to the potatoes and stir in the salad sauce. Let it flow. Season as needed.