Place the sweet potato in a pan of boiling water and cook for 15 minutes, or until tender. Meanwhile, heat the oil in a pan and quickly fry the chicken strips for 2-3 minutes. Remove from the pan. Add the onion and carrot to the same pan and fry for 2 minutes, then add the garlic and ginger and cook for 1 minute. Tip in the curry powder, stir round the pan then add 750ml of the boiling water with the chicken stock pot, tomato purée and lentils. Cover the pan and boil for 15 minutes until the vegetables are tender and the liquid has almost been absorbed. Add the chicken strips back to the pan along with the chopped coriander. When the sweet potatoes are cooked, drain and mash them with the yogurt, cumin and season well with salt and pepper. Spoon the chicken and lentil mix into one big dish then top with the sweet potato mixture. Scatter with the extra coriander and toasted cumin seeds.