Pour the tomato and herb sauce into a large saucepan and stir in the chopped tomatoes, harissa paste, chargrilled Mediterranean vegetables, green beans and half the chopped parsley. Simmer over a low heat for 5 minutes.
Meanwhile, put the couscous in a heatproof bowl and pour over 350ml boiling water. Cover with cling film and set aside 5 minutes. Now add the falafels and chickpeas to the simmering tomato mixture and cook for 5 minutes. Fluff up the the couscous with a fork and stir through the remaining chopped parsley. Season with freshly ground black pepper.
Serve the tagine with the couscous and topped with extra harissa paste, if you like it extra spicy.