Cook the potatoes until they are firm to the bite, peel them and let them cool. Then cut or dice into slices and place in a large bowl.
Now dice the onions, apples and cucumber and add them. Pluck the dill into the bowl, cut the herring fillets into bite-sized pieces and add to the potatoes with the capers. Then mix the mayonnaise (or light salad cream) with the cranberries, the cream and a little mustard, pour over the salad and mix well. Season to taste with mustard and salt.
The salad should sit for at least 3 hours, but preferably overnight.