While the squash cooks, melt the butter with the remaining oil in a large saucepan, then add the onions, garlic and ¾ of the chilli. Cover and cook on a very low heat for 15-20 mins until the onions are completely soft.
Tip the squash into the pan, add the stock and the crème fraîche, then whizz with a stick blender until smooth. For a really silky soup, put the soup into a liquidiser and blitz it in batches. Return to the pan, gently reheat, then season to taste. Serve the soup in bowls with swirls of crème fraîche and a scattering of the remaining chopped chilli.