Heat the butter in a pan and sweat the onion without colouring for about five minutes. Add the cauliflower florets and leaves, potato, garlic and chilli if using and cook for a further five minutes until softened. Add the fish sauce and stock and bring to the boil. Simmer for about 20 minutes until the cauliflower is completely tender.
Liquidise in a blender, then return to the pan. Pour in the coconut milk, season and reheat. Serve with lime wedges.