Peel the potatoes and cut them into large pieces which are no more than 2 cms thick, cutting lengthwise to maximise the flat surface area available for contact with the roasting pan. Put into a saucepan, cover with water, put the lid on and bring to the boil. Lower heat and boil gently for about 5 minutes. While they are boiling pour a couple of tablespoons of oil onto a flat baking tray and place on the lowest rack of the oven to heat for a couple of minutes at 425°. Once the potatoes have boiled for 5 minutes, remove them from the heat, and drain off the water and let the steam escape with the lid off.

Remove the heated baking tray and oil from the oven, turn the hot drained potatoes onto the tray (be careful, they will sizzle and the hot oil may spit). Distribute the potatoes evenly in a single layer, turning each piece until all sides are coated with oil. Place the tray on the lowest rung of the oven and cook at 425° for about 30 minutes, turning the potatoes after every 10 minutes until all sides are crunchy and golden brown.