Preheat the oven to 200°C/Fan 180°C/Gas 6. Place the potatoes between the handles of 2 wooden spoons and cut them at 2mm intervals until the knife hits the handles, so you don’t slice them all the way through.
Pop the potatoes onto a roasting tray. Drizzle over the butter and oil, sprinkle over the rosemary and season. Roast in the oven for 45 minutes–1 hour. Serve with fish, lamb chops or chicken breast.