Smoked mackerel salad
Put the potatoes in a pan of cold water, cover and bring to the boil. Cook for 10 minutes or until tender. Drain well, tip back into the pan and add 1 tablespoon of the oil. Season well with salt and pepper. Cover the pan and shake well to mix. Spoon the potatoes into a large bowl.
Add the mackerel, tomatoes and cucumber, and top with the watercress and a pinch of salt and pepper.
Mix together the horseradish, remaining oil and the vinegar in a bowl, and season well with salt and pepper.
Pour the dressing over the salad. Toss gently, and serve