Wash the potatoes and bake in the oven until soft.
Finely chop the cucumber and onion. Mix mayonnaise, sour cream and mustard, add to vegetables, mix and season with pepper. Boil and peel the eggs and cut them into 2 halves. Cut the herring fillet into small slices.
Cut the potatoes into 2 parts, put the cucumber-onion mixture on top, then put the herring pieces and half an egg. You can serve it on slices of black bread or just like that.