For the mackerel, blend all of the ingredients for the curing mixture in a food processor until slushy and well combined.
Place the mackerel fillets into a large ovenproof dish, spoon over the curing mixture and mix until all sides of the mackerel fillets are covered in the curing mixture. Set aside for two hours.
Meanwhile, for the herb mayonnaise, blend the egg yolks, Dijon mustard and white wine vinegar in a food processor until smooth and well combined.
With the motor still running, add the herb oil one drop at a time, until the mixture has thickened. Drizzle in a little water if needed, to loosen the mixture. Season the mayonnaise with a pinch of salt. Chill in the fridge until needed.
For the salad, mix together the celery, walnuts, apple batons and gooseberries in a bowl. Set aside until needed.
Cover the bottom of a heavy-based frying pan with olive oil and heat until it is just shimmering in the pan. Remove the pan from the heat and add the onion slices (make sure the onions are completely covered by the oil). Set aside for 10-20 minutes, or until the onions have softened in the warm oil. Drain well before serving.
When the mackerel fillets have cured, remove them from the curing mixture and rinse gently. Pat dry and cut each mackerel fillet into three pieces.
Heat a little oil in a frying pan over a medium heat. Add the pieces of cured mackerel, in batches if necessary, and fry for 20-30 seconds on each side, or until lightly browned on both sides. Carefully remove the mackerel pieces from the pan using a fish slice.
To serve, divide the rocket leaves equally among four serving plates. Mix the salad with the herb mayonnaise until well combined and spoon the salad on top of the rocket leaves. Place two mackerel fillets on top of each portion of salad. Garnish each plate with the softened onions.