Remove all ingredients from the glasses and drain well. Cut the onions into small cubes. Also cut the potatoes, apples and beetroot into small pieces. Halve the Bismarck herrings lengthways and cut across into strips.
All ingredients come in a large earthenware pot or bowl. Add the horseradish, mustard, mayonnaise and yoghurt and mix everything together well. Season to taste with salt and pepper. Put in a cold place and let it steep for at least 2 days. Stir every now and then.
To serve, chop the parsley and add it with the 2 quartered eggs and the walnuts, stir and decorate with the remaining, sliced eggs.
The herring salad can be eaten at any time of the day. If it is too dry, just add more yogurt.