Heat the oil in a saucepan over a medium heat, add the lamb and brown for 3–4 minutes, then remove from the pan.
Add the onion, carrots and garlic to the pan and cook for 2–3 minutes, then stir in the spices. Return the lamb to the pan and stir to coat with the spices. Pour in the stock, stir in the tomato purée and sugar and simmer for 12 minutes, stirring occasionally.
Meanwhile, cook the sweet potatoes in a large saucepan of boiling water for 12–15 minutes until tender. Drain and mash with the butter and salt and pepper.
Stir the chopped coriander into the lamb, spoon into an ovenproof dish and top with the mashed sweet potato.
Bake in a preheated oven, 200˚C (400˚F), Gas Mark 6, for 10–12 minutes and serve.