For the sauce, mix all the ingredients together and put into your slow cooker. Put on low.
Mix the rub ingredients together and rub into the beef. Put this in the slow cooker with the sauce. Leave for 10 - 12 hours until a fork goes in with no resistance. Shred with forks and add the spring onions. Keep warm until needed.
Preheat the oven to 170C. Rub a little oil over the sweet potatoes, then sprinkle with salt and the cumin seeds. Pop in the oven and cook for 45 minutes - 1 hour until tender.
Split the potato, add the beef and top with sour cream, lime and coriander.