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Heat the oven to 180C/160C fan/gas 4. Scrub the sweet potatoes clean, put on a baking tray and prick all over with a fork. Drizzle each with a little oil and sprinkle with a pinch of sea salt flakes. Put in the oven for 45 mins to 1 hr, depending on their size, until crisp on the outside, and they're pierced easily with a small knife. Serve as a side with butter, or add a filling of your choice.