Preheat the oven to 200°C. Put the zucchini, capsicums and red onions in a roasting tin, spoon over the olive oil and toss together to coat the vegetables. Put the tin in the oven and roast for 30 minutes, turning the vegetables after about 15 minutes.
Meanwhile, combine all the marinade ingredients in a shallow dish. Add the lamb, turn to coat, then cover and set aside. Put all the dressing ingredients in a screw-top jar, shake together and set aside.
Stir the vegetables once they have roasted for 30 minutes, arrange the lamb on top and pour over the marinade. Return to the oven and continue roasting for a further 30 minutes until the lamb is cooked but still juicy. (You can prepare ahead up to this stage if more convenient and chill for up to 24 hours.)
Put the couscous in a heatproof bowl and pour over enough boiling water to cover. Set aside, covered, for about 5 minutes until the water is absorbed and the grains are tender. Fork in the herbs, olives and spring onions.
Transfer the lamb to a plate and allow to rest for 10 minutes. Stir all the roasted vegetables into the couscous, then thinly slice the lamb and lay it on top of the couscous. Shake the dressing and pour it over the salad. Toss together, then serve warm.
If you have time, you can marinate the lamb overnight.