Mix tartar sauce, yoghurt, mustard and cream in a large bowl and season.
Peel and dice shallots and add to the bowl for marinade. Clean the spring onions, cut into rolls and also fill the bowl. Partially peel and quarter the cucumber, remove the seeds, cut into cubes and add to the marinade.
Stir everything, add the diced egg and carefully fold in. Peel the potatoes, cut into cubes, mix with the ingredients and season again to taste.
Tip: Possibly stir in a little more stock so that the marinade becomes thicker.
Let the salad steep in the refrigerator for at least 2 hours. Then it can be served.