Mix together the harissa, cumin, garlic, lemon zest, juice and oil. Rub it all over the chicken breasts and leave them to marinate in the fridge for at least one hour (or overnight if you have the time).
Bring the chicken to room temperature, preheat the grill and line the grill pan with foil. Season and cook the chicken for 10-12mins turning once until just cooked through with no pink showing and lightly charred. Wrap in the foil and leave to rest for 5mins to relax and release its juices.
Serve the chicken with the couscous and salad leaves, pouring over any pan juices.
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