Heat oil in a saucepan over medium heat. Add onion, garlic, spice mix and some salt and pepper, and cook, stirring, for 5 minutes until soft. Stir in chickpeas, tomato and stock. Bring to the boil, then cover, reduce heat to low and simmer for 10 minutes. Add the zucchini, beans and figs and cook for a further 5-8 minutes until the vegetables are tender.
Meanwhile, place the couscous in a large heatproof bowl and pour over 2 cups (500ml) boiling water. Cover the bowl with a clean tea towel. Set aside for 5 minutes, then fluff up with a fork.
Divide couscous and vegetable tagine among plates, garnish with coriander then serve with harissa, if desired.