Fry the onion in the oil for 5 mins until softened. Add the sherry and squash and sizzle for 1-2 mins. Pour in the stock, then cover and simmer for 20 mins until the squash is tender when pierced with a knife.
Whizz in a food processor until smooth. Will keep in the fridge for 2 days, or freeze for 6 weeks. When ready to eat, reheat until bubbling and serve in small portions topped with seed-bread croûtons (see above right) and a parsley sprig.