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Heat oven to 180C/160C fan/gas 4. Heat the oil in a shallow flameproof casserole. Fry the bacon until crisp and golden, then add the onion and leeks. Cook until they just start to soften, about 2 mins, then stir in the zest and butter. Once melted, lay the mullet, skin-side up, on top of the leek ragout. Season with flaky sea salt and bake uncovered for 12 mins.
Meanwhile, put the grapefruit segments on a heavy baking sheet and blowtorch or grill until the segments are charred with a toasty edge. Remove the fish from the oven, top with the charred segments and serve.