In a sense this could be a Thai recipe, combining salt (herring) with sweet (mayo, beetroot) and sour (gurkins, vinegar, capers, Granny Smith)).
Basic ingredients are equal parts of boiled potatoes and beetroot, in 5x5 mm dices. The dressing is made from mayonaise and horse radish, but I added a little apple vinegar.
A quarter Granny Smith, finely diced, slices of a shallot, spoonful of capers and finely cut gurkin or cornichons. Spoonfull of dill.
Mix and taste, leave in the fridge for at least a couple of hours. Taste again, now you can always add more of one of the ingredients. Or if sour is not your favourite use a sweet apple and leave out the vinegar.
Mine was served with samphire. I like the raw, salted herring on top, but others mix the herring with the salad.