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Preheat the oven to 250°C/500°F/Gas 9.
Scrub the squash with a vegetable brush. Cut it in half, remove the seeds and then cut the flesh into chunks; you should have about 500g.
Put the onions in an oiled roasting pan together with the squash, sweetcorn and the halved garlic bulb. Roast in the middle of the oven for 30 minutes or until the vegetables have started to brown and the squash has softened.
Transfer the roasted vegetables to a large pan; if you like, set aside a handful of sweetcorn to garnish the finished soup. Cut off the coriander leaves and place the stalks in the pan, along with the coriander seeds, chilli and 1 litre of water. Using a stick blender, blend until smooth.
To serve, add salt and pepper to taste and bring to the boil. Serve the soup in bowls, topped with the coriander leaves and reserved sweetcorn. It’s also nice with toasted sunflower seeds and sesame seeds, with some good bread on the side.