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Place the potatoes in a pan of cold, salted water and bring to the boil. Turn down the heat and simmer for exactly 5 mins. Tip into a colander, then leave to drain for a good 10 mins to ensure they are really dry, before gently tossing in the colander to rough them up slightly.
Heat oven to 200C/180C fan/gas 6. Heat the fat in a sturdy, roomy roasting tin on the hob until it starts to shimmer. Carefully lay the potatoes in a single layer in the fat and leave for 1 min to start to crisp. Turn them until completely coated in fat, then transfer the tin to the oven. Leave undisturbed for 40 mins to crisp up, then turn them again, adding the garlic if you like, before returning them to the oven for 20 mins or until completely crisp and golden.