Add the oil and onions to a heavy bottomed pan and fry for 3 minutes until softened then add the garlic and cook for a minute and add the ginger and cook for another minute.
Add the carrot, sweet potato and Thai curry paste and fry for a minute then add the coconut milk, stock and fish sauce and cook for 30 minutes.
Take off the heat, leave to cool for 5 minutes and then blend. Stir in the lime juice, serve and scatter over the coriander leaves.