Put the harissa and lemon rind and juice in a large heatproof bowl and add the couscous and boiling water. Stir, cover and leave to stand for 5 minutes. Season to taste and fluff with a fork.
Meanwhile, heat the oil in a large frying pan, add the vegetables and stir-fry over a moderate heat for 3–4 minutes. Cut the preserved lemons into quarters, remove the flesh and discard, then roughly chop the skin.
Stir the vegetables and preserved lemon skin into the couscous with the chickpeas and coriander. Season to taste and serve warm with a spoonful of yogurt and a drizzle of harissa.