Half fill a medium-sized saucepan with water and add a generous pinch of salt. Scrub the potatoes under running water, add them to the saucepan, bring the water to the boil and boil the potatoes in their skins for 25 - 30 minutes.

Rinse the herring in cold water, pat dry and cut it into squares. Cut the onion into rings. Peel, quarter, core and dice the apple. Drain the beetroot in a sieve and cut it into cubes, reserving 2 tablespoons of the juice. Slice the cucumber.

Beat the mayonnaise with the beetroot juice, yogurt, vinegar and pepper. Drain the potatoes, leave them to cool, remove the skins and slice. Gently mix the potatoes with the herring, onion rings, apple and beetroot in a large bowl.

Fold the dressing into the salad, cover the bowl and leave to stand for a few minutes before serving. Season to taste with salt.