Sweet potato soup

Cook the sweet potatoes in water for five to 10 minutes or until soft. Drain and set aside.
In a pan, heat the oil over medium heat, add the ginger, cumin, chillies and lemongrass and sauté for three minutes.
Remove the lemongrass and place the sweet potatoes, spice mixture and 100 ml of the stock in a food processor and purée until smooth.
Spoon the sweet potato mixture into a saucepan, add the remaining stock, coconut milk and sugar and heat until the mixture just comes to the boil.
Remove from the heat, stir in the coriander, season with salt and pepper and serve hot.
To garnish, stir turmeric into a little soup and dot each bowl with a little of the mixture.