Spoon the oil into a large roasting tin and place in the oven to heat up.
Drain the potatoes into a colander. Shake the potatoes to dry and roughen the edges so to give them a crispy coating.
When the oil is sizzling, remove from the oven and carefully add the potatoes to the hot roasting tin.
Spoon the oil over the potatoes for even covering and sprinkle with salt and rosemary.
Return to the top of the oven for 45-50 minutes, basting and turning once or twice during cooking, until golden and crispy.