Heat the coconut oil in a frying pan over a medium heat. Add the onion and fry for 6 minutes, until starting to soften. Turn the heat down slightly, add the ginger, garlic, chilli and spices and cook for another minute, stirring.
Transfer the onion mix to the slow cooker pot with the sweet potatoes, canned tomatoes, and hot water, then stir until combined. Cover and cook for 3–3½ hours on low or 2½–3 hours on high, until the sweet potato is tender and starts to break down in places. Turn off the heat, stir in the peanut butter and lime juice, without breaking down the sweet potato too much, then season with salt and pepper to taste. Leave to sit for 5 minutes to heat through.
Serve topped with the coriander, peanuts and extra chilli, with the chapattis on the side for scooping up the sauce and lime wedges for squeezing.