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Heat the 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 the butter with the remaining 1 tbsp olive oil in a large saucepan, then add the onions, garlic clove and three-quarters of the 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.