- Preheat oven to 200°C/180 °C fan/gas mark 6.
- Heat one tablespoon of the oil from the olives in an ovenproof saucepan. Add 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 black pepper. Bring to the boil, then add the olives.
- Put the fish onto the sauce with the skin side down, 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 to squeeze over the fish.