Dice the apples and pickled cucumbers into small pieces, and finely chop the onions. In a large bowl, mix the sour cream with the yogurt, mustard, coarsely ground pepper, and mustard seeds. Season with salt, pepper, and sweetener to taste. Stir in the apple pieces, cucumber, and onions until well combined.
Drain the salted herring fillets and place them in a bowl. Pour the prepared sauce over the herring, making sure it is completely covered. Cover the bowl and refrigerate for 1 hour.
Arrange the freshly cooked jacket potatoes on serving plates. Top with the marinated herring and its sauce, and serve with the canned beetroot on the side.