Place all the ingredients for the harissa in a small food processor or blender and pulse until smooth. Mix the harissa paste, yogurt and coriander together in a shallow dish. Coat the swordfish with marinade, cover and leave in the refrigerator overnight. Place the swordfish steaks onto a grill rack and cook on the highest level under a preheated grill for 2-3 minutes each side until cooked, depending on the thickness of the steaks. Serve with couscous or Waitrose Red Leaf Salad. Variation: Cut the swordfish steaks into 2.5cm (1in) cubes and marinate as above. Soak 8 wooden skewers in water for at least an hour. De-seed 1 red pepper and 1 yellow pepper and cut into chunks. Peel and quarter 1 red onion and separate the petals. Thread the skewers with cubes of swordfish, pieces of pepper and onion. Cook under a preheated grill for 4-5 minutes, turning frequently. NOTES : Swordfish is a meaty fish that keeps its shape well when cooked. Here it is marinated in yogurt and harissa, a traditional Middle Eastern mixture of spices, that gives it a wonderful flavour.