Heat a suitable pan and add the curry powder and dry fry for two minutes, then add the oil and the onions and cook for two minutes. Add in the apple, garlic, coriander and the ginger. Cook out for about five minutes, stirring occasionally.
Add the sweet potato to the pan with the stock, lentils and milk. Bring to a gentle simmer, cover and simmer for 20 minutes, then remove from the heat.
Blend until smooth, check for taste and flavour. Stir in the lime juice.