Begin by measuring the oil into the roasting tin, then pop it into the oven to heat through. Wash the potatoes but don't scrape the skins off, then cut them into cubes of roughly ½ inch (1 cm). Place them in a clean tea cloth and dry them as thoroughly as you can, then transfer them to a large plate. Remove the tin from the oven, place it over direct heat the oil needs to be very hot then carefully slide the potatoes straight into the hot oil. Turn them around to get a good coating of oil, sprinkling in the garlic and rosemary as you go. Return the tin to the oven and roast for 30-40 minutes or until the potatoes are golden brown and crisp. Season with salt and pepper before serving.
