Heat oven to 180C fan. Cut the squash into large cubes, about 4cm/1½in across, then toss in a large roasting tin with half the olive oil. Roast for 30 mins, turning once during cooking, until golden and soft.
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.