Preheat the oven to 200°C/gas mark 6. Prepare the sweet potatoes by slicing in half lengthways, and rubbing with 1.5 tsp of vegetable oil. Arrange in a baking tray cut side up and bake for 35 minutes.
Meanwhile, heat the coconut oil in a medium-sized pan over a medium heat. Add the onion and garlic. Sauté for 6-8 minutes until soft, then add the cumin, cinnamon and chipotle flakes. Continue to cook for a further 1 minute, then add the black beans, chopped tomatoes, brown sugar, red wine vinegar and 200ml water. Bring to the boil, then simmer uncovered for 10 minutes. Season to taste.
Juice 2 of the limes and zest 1 lime. While the beans are cooking, blitz the tahini, lime juice, lime zest, coriander and 50ml warm water in a mini blender until smooth. Let the baked sweet potatoes cool slightly before scooping out the insides, leaving a thin amount on the skin. Fill the sweet potato with the chilli and drizzle with the tahini. Sprinkle over the remaining coriander and serve with extra lime wedges.