Preheat the oven to 200C/392F/Gas 6. Prick each potato all over and rub with a little of the oil. Place them directly on the oven shelf and bake for 40-45 minutes until they feel slightly soft when squeezed, then leave to cool.
Cut each potato in half lengthways and then into quarters. Scoop out the flesh, retaining for another use such as mash, and leaving a layer of potato at least 5mm/½in thick on the skin.
Brush all over with the remaining oil and arrange in a single layer, skin-side down, on a wire rack set on a roasting tin.
Season generously and sprinkle over the rosemary.
Bake for 30 minutes, turning half way through, until crisp and golden brown. Serve hot.