Drain the tin of tuna in water well in a sieve. At the same time, rinse the jarred white beans under cold running water and let these drain well too. Shred the quarter red onion very finely and chop the fresh parsley into rough pieces.
Put the drained tuna into a large mixing bowl and pull the fish into smaller pieces with a fork. Add the white beans, the finely shredded red onion and the tablespoon of capers to this. Scoop in the mayonnaise made with olive oil, or alternatively the thick Greek yoghurt, and fold everything together thoroughly until everything is well mixed.
Then stir the freshly chopped parsley through the tuna mixture. Make sure the herbs are distributed evenly throughout the salad and press the whole thing down lightly with the back of a spoon so that the flavors can mix well.
Distribute a generous amount of arugula over a plate or shallow bowl to create a base. Neatly scoop the finished tuna salad with white beans into the middle on this bed of arugula and serve the dish immediately.