Put the lamb in a large shallow dish. Blend the parsley, mint, oregano, garlic, chilli, capers and 150ml/5fl oz oil in a food processor. Pour two thirds of the mixture over the lamb, cover with clingfilm and marinate in the fridge for 24 hours. Reserve the remaining mixture in the fridge, covered.
When you are ready to cook, preheat your barbecue to hot.
For the couscous, melt the butter with the oil in a heavy-based saucepan over a medium heat. Add the couscous and fry until dark and well toasted. Pour in 300ml/10fl oz cold water and cover with cling film, then set aside.
Mix the olives with the remaining 100ml/31/2fl oz oil, cover and set aside. For the salad, put the tomatoes in a shallow serving dish, then sprinkle over the vinegar, basil and a pinch of salt. Cover and set aside.
Cook the lamb on the barbecue for 4 minutes on each side, or until well browned but still pink in the middle. Transfer it to a serving plate, drizzle with oil and leave to rest for 20 minutes.
To finish the couscous, toast the almonds in a hot frying pan for 2-3 minutes. Fluff the couscous up using a fork, mix in the almonds, a drizzle of oil and a spoon of the reserved salsa verde and season with salt and pepper.
Arrange the tomatoes and couscous on plates and garnish with the olives. Add a quenelle of the salsa verde. Carve the lamb and divide equally between the plates. Spoon on the fromage frais and drizzle with the resting juices.