- Preheat the oven to 190C, 375F, Gas Mark 5 and lightly grease a shallow ovenproof dish.
- Heat the oil in a saucepan, add the onions and garlic and sauté for 5 minutes.
- Add the tomatoes, wine, parsley, salt and pepper, bring to the boil then reduce the heat a little and simmer for 15 minutes.
- Spread half of the sauce in the bottom of the baking dish, place the mackerel on top, cover with the remaining sauce and bake for about 35 minutes or until the fish is cooked through. Serve hot.