Toss together the squash and half the oil, then season. Place on a baking sheet and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 30 minutes, turning once, until soft and golden brown.
Meanwhile, heat the remaining oil in a large saucepan, add the onion and cook over a low heat for 7–10 minutes until soft. Add the garlic and half the chillies and cook for a further 1 minute.
Pour in the stock and bring to the boil, then reduce the heat and simmer for 5 minutes. Add the cooked squash and simmer for a further 5 minutes.
Whizz the soup until smooth in a blender or using a hand-held stick blender, then stir in 3 tablespoons of the soured cream and season.
Ladle the soup into bowls. Spoon over the remaining cream and top with the remaining chillies.