Preheat the oven to 180°C, fan 160°C, gas 4 and heat the oil in a large roasting tin for 5 minutes until really hot. Meanwhile, bring the potatoes to the boil in a pan of salted cold water, then simmer for 5 minutes. Drain well, return to the pan and shake over a low heat to rough up.
Carefully add the potatoes to the tin. Turn to coat, then roast for 50 minutes, turning occasionally, until crisp. Add a handful of fresh sage leaves, then roast for a further 10 minutes.