Preheat oven to 200°C. Place 1 potato, cut-side down, on a board and make thin, evenly spaced cuts at 5mm intervals, about two-thirds of the way through.
Place in a bowl. Repeat with remaining potatoes. Add oil, garlic and rosemary to potatoes. Season with salt and pepper and toss to combine.
Place in a roasting pan. Bake in oven, turning occasionally, for 1 hour or until golden brown and crisp. Transfer to a serving bowl to serve.
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