Heat a tbsp oil in a pan and cook the onion, garlic, lemongrass and ginger for 5 minutes. Add the carrots and cook for another 5 minutes. Mix the coconut milk in the tin in case it has separated then take out 2 tbsp and keep to finish the soup. Add the rest of the tin to the carrots with the stock. Simmer until the carrots are tender then whizz with a stick blender until smooth. Stir in the lime juice and zest then spoon into bowls and drizzle over the coconut milk to finish.