Heat the oil in an oven proof casserole, add the onion and cook gently for 5 minutes until softened. Add the ginger and garlic and continue cooking for 2 minutes
Add the spices and green chillies, mix well and cook for 1 minute.
Add the diced mutton and brown on all sides, ensuring the meat is well coated with the spice mixture.
Stir in the chopped tomatoes, stock and salt. Bring slowly to a simmer, cover with a tight fitting lid. Transfer to the oven and cook for 2 hours. Remove the dish from the oven, stir in the sweet potatoes and cook for a further 45 minutes to 1 hour, or until the mutton is tender and the sweet potato is cooked.