Place a steaming basket into a wide-bottomed pan, fill with water until it is just below the bottom of the basket, add the beans, salt and cook over a medium heat, covered, for approx. 25 mins. until soft. Remove and allow to cool a little.
For the dressing, mix the mustard with all the ingredients up to and including savory in a bowl, season. Add the potatoes and beans to the dressing and carefully combine. Top the salad with the fish pieces.