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Put the mackerel into a shallow dish, splash over the marinade and scatter with half the spring onions. Set aside. Put the rice into a pan, cover with a fingertip of water, season with salt, then bring to the boil. Turn down heat, stir, then cover and simmer for 10 mins until all of the water has disappeared. Take off the heat, cover and leave to steam for 5 mins.
Mix the cucumber with the remaining onions, spinach leaves and beansprouts. Stir together the sugar and vinegar with some seasoning. Lift the fish out of the marinade and put onto 2 plates. Drizzle with more marinade from the dish, if you like. Spoon the rice alongside, then toss the salad with the dressing and serve.