Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the onion and fry until softened. Stir in the curry paste, galangal and garlic and cook for 1 minute, then mix in the squash.
Pour in the coconut cream and stock, then add the soy sauce and bring to the boil, stirring. Pour into the slow cooker pot, cover with the lid and cook on low for 7–8 hours or until the squash is tender. (You may find that the coconut cream separates slightly but this will disappear after puréeing.)
Purée the soup while still in the slow cooker pot with a stick blender. Alternatively, transfer to a liquidizer and purée, in batches if necessary, until smooth, then return it to the slow cooker pot and reheat on high for 15 minutes.
Reserve a few sprigs of coriander for garnish, chop the rest and stir into the soup. Ladle the soup into bowls and garnish with the reserved coriander sprigs.