Cut 1 peeled and deseeded butternut squash into large cubes, about 4cm/1½in across, then toss in a large roasting tin with 1 tbsp of the olive oil.
Roast for 30 mins, turning once during cooking, until golden and soft.
While the butternut squash cooks, melt 1 tbsp butter with the remaining 1 tbsp olive oil in a large saucepan, then add 2 diced onions, 1 thinly sliced garlic clove and ¾ of the 2 deseeded and finely chopped red chillies.
Cover and cook on a very low heat for 15-20 mins until the onions are completely soft.
Tip the butternut squash into the pan, add 850ml hot vegetable stock and 4 tbsp 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.