For the potato salad, wash the potatoes and cook in water with skin. Drain the potatoes, let cool and peel. Cut four of the gherkins into very fine cubes.
Peel the onion and cut into fine cubes. Peel the apple, remove the core and also cut into small cubes. Mix the apple, cucumber and onion cubes together.
Add the mayonnaise and yogurt and mix with the cucumber water. Season to taste with salt and pepper (it should be very strongly seasoned, as unseasoned potatoes are still to come).
Cut the peeled and cooled potatoes into mung-sized cubes and add them to the salad dressing. Mix together and let stand for a few hours.
Season the salad again and garnish with the tomatoes and the remaining gherkin.