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To make the dressing, mix the oil with the lemon zest and juice, capers, mustard, some seasoning and 1 tbsp water. Don't add the parsley yet (unless serving straight away) as the acid in the lemon will fade the colour if they are left together for too long.
Heat the oven to 220C/200C fan/gas 7. Line a baking tray with baking parchment and put the fish, skin-side up, on top. Brush the skin with oil and sprinkle with some flaky salt. Bake for 7 mins or until the flesh flakes when tested with a knife. Arrange the fish on warm serving plates, spoon over the dressing and scatter with extra parsley leaves, if you like.