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Put the couscous into a large bowl, pour over 150ml boiling water, then cover with cling film. Set aside for 10 mins.
Meanwhile, mix the horseradish into the yogurt and season. When all of the liquid has been absorbed into the couscous, uncover it, then spread it up and around the sides of the bowl to help it cool more quickly.
When couscous is quite cool, add the cucumber, herbs, onion, lemon juice and oil and toss well. Add the beets and toss briefly. Pile the couscous onto plates, flake a mackerel fillet into a few big pieces alongside each serving (discarding the skin), then serve with a big dollop of the horseradish yogurt. Pack the remaining couscous and mackerel into a lunchbox for the next day.