Boil the potatoes in a large pan of water. After 15 minutes, add the beans and cook for 5 minutes. Drain then plunge into cold water.
Cut the cucumber into ribbons with a vegetable peeler and finely shred the lettuce, then put into a bowl with the watercress. To make the dressing, whisk the ingredients together and set aside.
Coat the trout in flour. Heat a little oil in a hot pan then fry the fish skin down for 5 minutes, then turn for 2-3 minutes. When cooked, flake the fish.
Add the potatoes and beans to the bowl, toss with the dressing and serve with the trout on top.