Wash the potatoes and cook in boiling salted water for about 20 minutes. In the meantime, clean and wash the beans and cut them into bite-sized pieces. Cook in boiling salted water for 15-20 minutes. Hard boil eggs for about 8 minutes, rinse with cold water and remove the shell. Leave to cool and cut into eighths.
Rinse potatoes cold, peel, let cool and cut into slices. Drain beans and let them drain. Peel onions, cut into very thin rings. For the marinade beat vinegar and 4 tablespoons of water. Season with salt, pepper and sugar, add oil. Add capers. Mix potatoes, beans, onions, eggs, olives and marinade carefully. Cover and leave to stand for approx. 30 minutes. Then season again. Drain tuna and cut into small pieces.
Add capers. Mix potatoes, beans, onions, eggs, olives and marinade carefully. Cover and leave to stand for approx. 30 minutes. Then season again. Drain tuna and cut into small pieces. Spread on the salad. Serve garnished with parsley