2. Meanwhile, place the sliced onions in a medium sized heatproof bowl. In a small saucepan, bring the vinegar, peppercorns, mustard seeds and sugar to a boil and simmer until the sugar is dissolved. Strain the vinegar mixture, pour it over the onions and set aside.
3. When the beets are cool enough to handle, peel them, cut them into wedges and toss them with the onions. Season with salt and pepper.
4. Transfer the beets and onions, along with their liquid to a serving bowl. Garnish the salad with sprigs of watercress and orange sections. Drizzle the salad with extra virgin olive oil and serve.