Heat the oil in a large saucepan and gently fry the onion, garlic and ginger for about 5 minutes until tender. Add the sweet potatoes and lemongrass and cook for a further 3 minutes.
Add the stock and bring to the boil. Reduce the heat and simmer, covered, for 20 minutes until the vegetables are tender.
Cool the soup slightly, then liquidise with half of the coconut cream and process until smooth.
Return the soup to the saucepan, add the remaining coconut cream.and season with salt and pepper. Heat through without allowing the soup to boil, and add the lime juice.
Ladle the soup into bowls and garnish with the lime zest.