Place the potatoes cut side down. Using a sharp knife, make cuts at 5 mm (¼ inch) intervals along the length of each potato almost through to the base, so that they just hold together.
Heat the oil in a roasting tin in a preheated oven, 200°C (400°F), Gas Mark 6, until hot. Add the potatoes to the tin and spoon over the oil evenly. Sprinkle with a little salt, baste well and roast the potatoes for 30 minutes.
Remove the potatoes from the oven, sprinkle with the sesame seeds, then return to the oven for a further 30 minutes until golden brown and crisp.