Preheat the oven to 200°C, 180°C fan, gas 6. Place the aubergines cut side up on a baking tray and score the flesh in a diamond pattern. Bake for 30 minutes until soft, adding the peppers for the last 10 minutes.
Meanwhile, put the stock, couscous and tomatoes in a large saucepan. Bring to the boil, cover then remove from the heat. Leave for 5 minutes to allow all the water to be absorbed then mix in the harissa, lemon zest and juice, lamb and coriander.
Score a 1cm border around the cut side of the aubergine halves and scoop out the flesh from the middle to make a shell. Chop the flesh, then add it to the couscous and stir. Season with black pepper and divide between the aubergine and pepper shells. Drizzle with olive oil and return to the oven for 15 minutes until piping hot. Serve with the yogurt in a bowl if you wish.