We make an emulsified vinaigrette with sherry vinegar, so it is served as a sauce for the tomatoes more than as a dressing, and it cuts through the oily flesh of the mackerel.
3 teaspoons Dijon mustard
7 tablespoons extra virgin olive oil
4 teaspoons sherry vinegar + pinch sugar if not sweet
3 tablespoons finely shredded basil
4 mackerel fillets, seasoned with salt and pepper
3 ripe vine tomatoes (300g), blanched in boiling water to remove the skins, deseeded and finely chopped
2 tablespoons finely chopped black olives
For the sherry vinaigrette, put the mustard into a small bowl and whisk in 5 tablespoons olive oil, then the sherry vinegar. Stir in the basil, set aside.
Place a large frying pan over a high heat and add the remaining olive oil.
When the oil is hot, but not smoking, add the fillets, flesh side down (in two batches if not enough room). Fry for 1–2 minutes until caramelised, then turn over for another minute or so. When just cooked, transfer to a plate.
Mix the chopped tomatoes and olives with the sherry vinaigrette and spoon over the fish. Serve immediately.