1 Heat the oil in a deep saucepan, add the sweet potatoes, carrots, ginger and cumin and fry over a high heat, stirring, for 10 minutes.
2 Add the stock, bring to the boil, then season with salt and black pepper. Cover with a lid and simmer over a low heat for 20-30 minutes.
3 Carefully scoop out half the vegetables into a bowl using a slotted spoon. Whiz the remainder in a processor or blender until smooth, then return to the pan.
4 Add the reserved vegetables, bring to the boil again.
5 To serve, garnish with a swirl of yoghurt and some chopped chives.