Preheat the oven to 200˚C/400˚F/gas 6. Peel and chop the sweet potatoes into bite-sized chunks, then place onto a baking tray. Sprinkle with a pinch each of cayenne, cumin, cinnamon, sea salt and black pepper, drizzle with oil then toss to coat. Roast for 45 minutes to 1 hour, or until golden and tender. Peel and roughly chop the onion. Halve, deseed and roughly chop the peppers, then peel and finely slice the garlic. Pick the coriander leaves, finely chopping the stalks. Deseed and finely chop the chillies. Meanwhile, put 2 tablespoons of oil in a large pan over a medium-high heat, then add the onion, peppers and garlic, and cook for 5 minutes, stirring regularly. Add the coriander stalks, chillies and spices, and cook for a further 5 to 10 minutes, or until softened and starting to caramelise, stirring occasionally. Add the beans, juice and all. Tip in the tomatoes, breaking them up with the back of a spoon, then stir well. Bring to the boil, then reduce the heat to medium-low and leave to tick away for 25 to 30 minutes, or until thickened and reduced – keep an eye on it, and add a splash of water to loosen, if needed. Stir the roasted sweet potato through the chilli with most of the coriander leaves, then taste and adjust the seasoning, if needed. Finish with a squeeze of lime or lemon juice or a swig of vinegar, to taste, then scatter over the remaining coriander. Delicious served with yoghurt or soured cream, guacamole and rice, or tortilla chips.