Wash the potatoes, then rub each with the coarse side of a sponge (do not peel them!). If they’re too big, cut them in half, then cook them in lightly salted water for about 15- 20 minutes until tender.
In the meantime, chop the chives and spring onion, mix them in a glass bowl with the oil and lemon juice. Season with salt & pepper.
Rinse the boiled potatoes under cold water, cut them into bite-size. Put them in a serving dish and toss with half of the dressing.
Rub the fish with oil, salt and pepper, then fry (with skin down first) in a frying pan or grill on both sides for 2-3 minutes. Rest for a couple of minutes on a plate. Remove & discard the skin, then flake the flesh into chunks.
Scatter the watercress ( if used) onto the potatoes, then add the fish and pour over the remaining dressing. Gently combine.
Serve at room temperature.