Heat oven to 180C. Heat 1 tbsp oil from the olives in an ovenproof pan. tip in teh onion and stir well then leave to cook for a minute or two. Add the tomatoes and some salt and pepper. Bring to the boil, then add the drianed olives.
Put the fish, skin side down, onto the sauce and drizzle with more oil from the olives. bake uncovered in the oven for 15 mins and serve with baked potatoes.