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Heat the oil in a large flameproof casserole or saucepan and gently fry the onion and leeks for 10-15 mins until well softened, stirring occasionally. Add the garlic and cook for 2 mins more.
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.
Remove the pan from the heat and ladle the stew into bowls. Scatter with coriander and the remaining nuts and top with yogurt, if using.