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In a large bowl, toss the onion with half the white wine vinegar and some salt. Set aside.
Boil the potatoes in a pan of salted water until tender. Drain well and put back in the pan to steam-dry for a few mins. Tip into the bowl with the onion and gently toss together so that the potatoes absorb the vinegar. Season and leave to cool.
In a small bowl, mix together the buttermilk, mayonnaise, remaining vinegar and the horseradish. Add some salt and fresh ground pepper, and mix well. Toss the potatoes in the mixture.
Boil the beans for 1 min in salted water, drain, rinse in cold water and dry. Divide the potatoes between two plates and top with the apple slices, beans, salmon and a large handful of watercress.