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Preheat the oven to fan 180C/conventional 200C/gas 6. Heat 1 tbsp of the oil from the olives in an ovenproof pan. Tip in the onion and stir well, leave to cook for a minute or two and then give it another good stir. Add the tomatoes and some salt and pepper. Bring to the boil, then add the olives.
Put the fish, skin side down, onto the sauce and drizzle over a splash more oil from the olive jar. Bake, uncovered, for 15 minutes until the fish is cooked. Sprinkle with chopped parsley and serve straight from the pan, with lemon wedges for squeezing over.