For the dressing
Preheat the oven to 200°C, gas mark 6.
Boil the potatoes until al dente then set aside to cool before cutting each in half. Heat the oil in a medium roasting dish in the pre-heated oven then arrange the halved potatoes and roast for 30-45 minutes, turning every now and then until crisp and golden. Wrap the whole garlic in tinfoil and place in the oven at the same time to roast.
In the meantime, make the dressing by mixing the olive oil, lemon zest and juice, chives and seasoning. Once the garlic has roasted, squeeze the caramelised cloves into the dressing and stir well. Serve the warm roast potatoes with the watercress, red onions and capers, and drizzle with the dressing.