Place all the vegetables in an ovenproof dish with the garlic and ginger. Pour over the oil, season well with salt and pepper and place in a preheated oven, 200°C (400°F), Gas Mark 6, for about 40 minutes until the vegetables are tender and nicely browned.
Meanwhile, tip the couscous into an ovenproof dish. Stir the salt into the measurement water and pour over the couscous. Stir once to make sure all the grains are submerged in the water, then cover with a clean tea towel and leave to stand for 10–15 minutes. Rake a fork through the couscous to break up the grains. Then, using your fingers, rub the oil into the grains until light, airy and any lumps are broken up. Toss in the cumin seeds, scatter over the butter and cover with a piece of damp greaseproof paper. Place in the oven for 15–20 minutes until heated through.
Pile the couscous in a mound on a shallow serving dish and spoon the roasted vegetables and cooking juices over and around the couscous. Garnish with the coriander and serve with lemon wedges to squeeze over it.