Heat the oil in a large pan over a medium heat and fry the onions with a pinch of salt for 6-8 mins until soft. Add the potatoes and cook for 2 mins more, then tip in the marrow and curry powder. Cook for 1 min. Pour in the stock, butter beans and most of the coconut milk (reserve a few tablespoons). Bring the mixture to the boil, then reduce the heat to medium-low and simmer for 20 mins until all the vegetables are tender.

Remove from the heat and leave to cool slightly, then transfer the mixture to a blender and blitz until smooth. Return the soup to the pan and warm through over a low heat, then spoon into bowls. Drizzle with the reserved coconut milk and scatter with the coriander.