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Heat oven to 220C/200C fan/ gas 6. Prick the potato all over with a fork and bake for 1 hr until it is golden outside and soft inside. Mix 1 tsp olive oil with the garlic salt. Cut a deep cross into the top of the jacket, drizzle the garlic oil into the cross and rub it all over the outside. Return to the oven and bake for 15 mins more until the edges are golden and crispy.
Meanwhile, crumble the feta into a bowl, add the yogurt and whisk together until creamy. Stir in the red pepper with a good grind of black pepper and spoon the whipped feta into the jacket. Sprinkle with the sumac, drizzle over the remaining 1 tsp olive oil and scatter a few torn basil leaves on top, if you like.