Wash the potatoes thoroughly and boil them in boiling water for about 20 minutes. Drain and allow to cool. Boil eggs in boiling water for about 10 minutes until hard. Drain, quench, peel and let cool off. In the meantime, pour cucumbers into a sieve, collect the stock and let the cucumbers drain. Peel and chop onions. Heat the oil in a pan. Fry the onions in it. Sprinkle with sugar and let it caramelize slightly. Remove from the heat and let it cool down.
Mix mayonnaise, yoghurt and mustard. Peel, quarter, core and cut the apple into pieces. Add 4 tablespoons cucumber stock, onions and apple to the mayo mixture. Puree everything finely. Season to taste with salt and pepper. Cut potatoes in half, mix with the sauce and arrange in a bowl.
Wash the tomatoes, drain and cut in half. Drain mozzarella and mix with pesto. Cut the eggs into pieces. Wash parsley, shake dry, remove leaves and chop coarsely. Season salad again and sprinkle with parsley. Add mozzarella, tomatoes, fried onions, eggs and cucumber to the potato salad.