Preheat the oven to 220ºC, fan 200ºC, gas 7. Peel the potatoes, cut into halves (or quarters for the larger ones) then parboil for 8 minutes. Drain and shake in a colander to roughen up the edges.
Heat the sunflower oil in a large baking tray then carefully tip the parboiled potatoes into the hot oil. Scatter over the unpeeled cloves of 1 bulb of garlic and the fresh rosemary leaves.
Toss to coat. Roast for 45 minutes, turning once, until the potatoes are crisp and golden.