Place the sweet potato in a pan of boiling water and cook for 15 mins, or until tender.
Meanwhile, heat the oil in a pan and fry the onion and carrot, for 2 minutes, then add the garlic and ginger and cook, stirring for 1 min more. Tip in the curry powder, stir round the pan then add 750ml of the boiling water with the stock cube, tomato purée and lentils. Cover the pan and boil for 20 mins until the vegetables are tender and the liquid has been absorbed. Stir in the chopped coriander.
When the sweet potatoes are cooked, drain and mash them with the yogurt and seasoning.
Spoon the lentil mix into one big or two individual dishes then top with the sweet potato mixture, scatter with the coriander and serve with broccoli, if you like.