Heat oven to 200C/180C fan/gas 6.
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.
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.