Preheat the oven to 200°C (400°F/ Gas Mark 6). Put the potatoes in a large pan with cold salted water. Bring to the boil and cook for 15 minutes until tender. Drain and leave to steam dry for a few minutes.
Arrange the potatoes in a large shallow baking tin, leaving enough space between each one so they have room to spread out a little. Using the back of a large spoon or potato masher, push down on each of the potatoes to make them crack open, then season.
Heat the oil and butter in a small saucepan, toss in the rosemary or thyme to coat and pour it over the potatoes. Roast for 1 hour, turning once and removing the rosemary or thyme, until the potatoes are crisp and golden. Pile into a warmed dish to serve.
