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Brush each fillet 2-3 times with a little of the olive oil and rub with the lime zest. Season and sprinkle with half the lime juice then marinate for 5 mins.
Heat the remaining olive oil in a large pan. Add half the onions and harissa, and stir over a medium heat for 2-3 mins. Stir in the couscous and toss to coat with the harissa. Add the hot stock, cover and remove from the heat. Stand for 5 mins, fluff with a fork and stir in the remaining spring onions, coriander, lime juice and some seasoning. Heat the grill.
Grill the mackerel for 3-4 mins on each side or until the flesh is opaque. Divide the couscous between 4 plates and slide a fillet on top. Serve with lime wedges.