Heat the sunflower oil for the lamb in a large casserole dish over a moderate heat until hot. Season the lamb pieces and sear, in batches if necessary, until golden brown in colour all over. Add all the lamb back to the dish, then add the sweet potato slices. Pour in 500ml of water, cover the dish with a lid and cook over a low heat for 45 minutes until the sweet potato and lamb are tender.
Meanwhile, prepare the sauce by toasting the spices in frying pan over a medium heat for 1 minute, stirring occasionally.
Add the sunflower oil to the pan and some seasoning and fry the spices for a few minutes. Add the yogurt to the pan and stir well until you have a smooth sauce. Simmer for a few minutes, then stir through the chopped coriander.
Add the broad beans to the lamb and sweet potato a few minutes before they are finished cooking. Remove from the heat and arrange the lamb and sweet potato in a serving dish.
Spoon over the broad beans then spoon the yogurt sauce all over. Serve immediately.
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