Green beans wash, clean and cook in boiling salted water for about 12 minutes. Rinse white beans and drain. Peel onions and cut into strips. Whisk vinegar and mustard. Season to taste with salt, pepper and sugar.
Fold in the oil. Drain tuna, pluck roughly. Place in a bowl with beans, onions, olives and capers. Add dressing, mix and marinate for about 20 minutes. Serve in small bowls
20 minutes waiting time