Dice the apples and cucumber, finely chop the onions. Mix the sour cream with yoghurt, mustard, pepper and mustard seeds and season with salt, pepper and sweetener. Stir in the apple and cucumber pieces and the onions.
Drain the herring, place in a bowl and cover with the sauce. Let it steep in the refrigerator for about 1 hour.
Serve the herring fillets with the sauce and serve with jacket potatoes and beetroot.