Put all the ingredients for the dressing in an empty jam jar with a quarter teaspoon each of salt and black pepper. Seal the jar, then shake vigorously and set aside for five minutes.
Place the red onion in a shallow bowl, pour over the dressing (removing the garlic) and leave to marinate. (This removes some of the onion’s astringency.)
Take a plate and cover it with kitchen paper, ready to soak up the oil from the mackerel later. Preheat the oven to 200°C/fan 180°C/Gas 6 and cut the pieces of ciabatta in half. Pop the bread in the oven for five minutes, so it warms up and is lightly toasted.
Spoon a couple of teaspoons of the dressing over the flesh side of the mackerel, then season the fish generously with salt. Heat two tablespoons of regular olive oil in a frying pan over a medium-high heat. Add the mackerel, skin side down, and fry for three to four minutes. Once the skin has begun to crisp up, flip the fillets and cook for another one to two minutes on the other side, until just cooked through.
While the fish is cooking, place the mint, carrot and rocket in the bowl with the onion and toss well.
When the mackerel is ready, transfer the fish to the plate lined with kitchen paper and begin assembling the sandwiches.
Drizzle the toasted bread with good-quality extra virgin olive oil. Then layer the mackerel on to the bread, followed by the tomatoes with a pinch of salt added, then the carrot mixture, then a couple of teaspoons of the dressing. Pop the top layer of bread on the sandwich and eat immediately!