Stir in the ground coriander, cumin, chilli and cinnamon. Cook for 2 mins, stirring occasionally. Season with plenty of ground black pepper. Add the chopped tomatoes, peppers, chickpeas, lentils, sweet potatoes, orange peel and juice, half the nuts and 400ml/14fl oz water and bring to a simmer. Cook for 15 mins, adding a splash of water if the stew looks too dry, and stir occasionally until the potatoes are softened but not breaking apart.