Wash the potatoes and cook them in salted water for 10-20 minutes, depending on their size. Then peel and let cool down completely.
For the mayonnaise, briefly whip the soy milk with the vinegar (preferably with a hand blender), slowly run in the oil while whipping, then add the mustard, salt and pepper.
Mix the mayonnaise with soy yoghurt, mustard and cucumber liquid, season with salt and pepper.
Peel the onion and cut into fine cubes, drain the cucumber and cut into slices or cubes. Core and peel the apple (organic apples also taste good with the peel) and also cut into cubes.
Cut the potatoes into not too small pieces and add to the cream with the other ingredients. Mix everything carefully, season to taste and let steep a little to taste.