Heat the oil in a saucepan, add the onion and fry gently for 5 minutes until softened. Stir in the curry paste, garlic and ginger and cook for 1 minute. Then mix in the squash, coconut milk, stock and fish sauce. Add a little pepper (don't add salt as the fish sauce is so salty) then bring to the boil.
Cover the pan and simmer for 45 minutes, stirring occasionally, until the squash is soft. Leave to cool slightly. Reserve a few sprigs of coriander for garnish then tear the remainder into pieces and add to the soup. Purée the soup in batches in a blender or food processor until smooth. Pour back into the saucepan and reheat, tearing in the reserved coriander sprigs. Ladle into bowls and serve.