1Saute the onion very gently, covered, in 2 tablespoons of the oil or butter for 5 minutes or until is tender.
2Add the ginger and the sweet potato and fry gently, covered for another 6-8 minutes.
3Add the apple, stir and cook for 2-3 minutes. Add the stock, coriander and salt and pepper. Bring to a simmer and cook, half covered, for 15-20 minutes or until the sweet potato and apple are tender.
4Put in a blender to form a smooth soup. Add some lemon juice and reheat. Add few spoons of sour cream or yoghurt.
5Ladle to a bowl and garnish with the chopped apples.