Prepare the vegetables and garnish before cooking the mackerel. Cut off the ends of the cucumber then use a swivel head peeler to slice from one end to the other, making long ribbons of cucumber like tagliatelle
Give the cucumber a quarter turn after you have peeled each length, so that you peel down to the core evenly and get narrow strips
Stop when you reach the seeds at the centre and set the ribbons aside in a bowl for now
Cut the chilli in half lengthways and remove the seeds, then cut the chilli into a fine dice or strips. Add three-quarters of it to the cucumber ribbons and reserve the remainder
Now prepare the spring onions. Trim off the root, pulling away any loose or damaged outer layers. Thinly slice the spring onion on an angle across the width
Heat a non-stick frying pan and add a generous film of olive oil. While waiting for the heat to rise, cut the limes in half, squeezing the juice over the cucumber and chilli mix along with two tablespoons of olive oil
Cook as many fillets at one time as fit comfortably in the pan without crowding
Sear the fish on the skin side until crisp, which should take about a minute and a half
Squeeze a little lime juice over the fillets then turn them over. Cook the fish for another minute then lift the fillets out on to a plate lined with kitchen paper
Arrange the cucumber mix in a long pile across the middle of four serving plates. Place 2 mackerel fillets on each mound then scatter the rest of the spring onion and remaining chilli over the top
Spoon any juice left in the bowl of cucumber over the fish then eat at once
