Place a potato between two wooden spoon handles. Slice downwards into 3mm intervals; the spoon handles will stop you from cutting all the way through.
If using garlic, in a small bowl, mix the garlic in with the olive oil.
Place the potatoes on a baking tray. Brush half the oil over the potatoes, pulling the slices gently apart so it gets inside. Season generously with sea salt.
Roast at 200°C (180°C fan / Gas 6) for 30 minutes.
Remove from oven, brush with the remaining oil, sprinkle rosemary leaves over the top and roast for another 15–20 minutes until intensely golden and crispy.