Wash the potatoes and cook for 15-20 minutes in boiling salted water. Drain, peel and leave to rest for a while. Cut one beetroot into small cubes, 2 into slices. Peel and finely dice the onion. Wash the apple and cut a thin slice out of the middle. Quarter the apple, remove the seeds and dice finely.
Chop nuts coarsely. Mix all prepared ingredients. For the marinade, mix egg yolk, mustard, salt, vinegar, stock, sour cream and yoghurt until creamy and mix with the potato salad. Wash the parsley, dab dry and chop finely. Arrange the pink potato salad together with the smoked mackerel and the beetroot cut into slices. Sprinkle with the parsley