Cook the potatoes in salted boiling water until tender. Drain and leave to cool a little then slice into bite sized pieces.
Take a large mixing bowl and whisk together all the ingredients for the dressing. Whilst the potatoes are still warm toss them into the dressing along with the spring onions.
Roughly chop the watercress, fold into the salad with the beetroot but don’t mix too vigorously otherwise the salad will go pink. Just before serving sprinkle over the cress and serve.