Combine cumin, paprika, garlic, Worcestershire sauce, 2 tablespoons of orange juice and half the oil in a screw-top jar. Season to taste, shake well. Divide the dressing between two shallow dishes, place lamb in one dish and mushrooms in the other and turn to coat in dressing. Cover and marinate in the fridge for 30 minutes.
Put couscous in a heatproof bowl, pour over 1½ cups boiling water. Cover and stand for 5 minutes. Add the parsley, remaining oil and orange juice, season to taste and stir with a fork to separate the grains.
Preheat barbecue plate on medium-high. For rare lamb barbecue 3-4 minutes on each side, or cook to your liking. Set aside in a warm place to rest for 10 minutes. While lamb is resting barbecue the mushrooms for 3-4 minutes each side until tender.
Thickly slice the lamb and mushrooms, drizzle with yoghurt and serve with couscous.
Servings 4