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.