Heat the oven to 220C/200C fan/gas 7. Arrange two wooden spoons on a work surface horizontally, then sit a potato between them. Cut a row of deep cuts, each a few millimetres apart, vertically down the length of the potato, stopping when you hit the spoons. (Make sure not to cut all the way through.) Repeat with the remaining potatoes.

Tip the potatoes into a shallow roasting tin with the oil, garlic, rosemary, 1 tsp sea salt flakes and 1/2 tsp freshly ground black pepper. Using your hands, rub the potatoes in the seasoned oil, ensuring they are evenly coated. Arrange cut-side up and roast for 1 hr until tender and golden, moving them around occasionally and basting in the oil.