If using wooden skewers, soak in water for 30 minutes so they don't burn.
Place the tuna in a shallow non-metallic dish. In a small bowl, combine the oil, cumin and lemon rind and pour over the tuna. Toss to coat, then cover and marinate in the refrigerator for 10 minutes.
Meanwhile, for the chermoula, place the coriander, cumin, paprika and cayenne pepper in a small frying pan and toast over medium heat for 30 seconds, or until fragrant. DO NOT BURN! Combine with the remaining chermoula ingredients and set aside.
Thread the tuna onto the skewers. Lightly oil your indoor grill or your barbeque and cook the skewers for 1 minute on each side for rare, or 2 minutes for medium.
Serve the skewers on a bed of couscous and drizzle with the chermoula.