Wash the potatoes thoroughly and cook them with their skins in boiling water for about 20 minutes. Drain and allow to cool. Boil eggs hard for about 10 minutes. Drain, quench, peel and let cool off.
In the meantime drain the mozzarella. Drain the cucumbers in a sieve and collect the stock. Peel and chop the onions and fry them in hot oil. Sprinkle 1 tsp. sugar over them and caramelise slightly. Remove from the heat and let it cool down.
Mix mayonnaise, yoghurt and mustard. Peel, quarter, core and cut the apple into large pieces. Finely puree apple, onions and 3 tbsp. cucumber stock with a hand blender. Stir into the mayo mixture. Season to taste with salt and pepper. Halve potatoes and mix with the sauce.
Wash the tomatoes, cut them in half. Mix mozzarella and pesto. Cut the eggs into pieces. Wash parsley, shake dry, pluck off leaves and chop coarsely. Season salad again, sprinkle with parsley. Serve the mozzarella, cucumber, tomatoes, eggs and roasted onions separately in small bowls with the potato salad, so that everyone can mix their favourite potato salad.