Fry the onion, lemongrass and most of the chilli in a large pan with a splash of water for 2-3 mins until softened - add more water if it starts to catch. Tip in the squash and stir. Cover with 1 litre water, bring to the boil and simmer for 15 mins until the squash is tender. Add lime juice, remove from the heat and blitz with a hand blender until smooth.
Pour in the coconut milk, season, then return to the heat to gently warm through. Ladle into bowls and serve with coriander and the remaining chilli.