Step 1

In a large bowl, toss the onion with half the white wine vinegar and some salt. Set aside.

Step 2

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.

Step 3

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.

Step 4

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.