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Heat the oil in a large pan, add the onion and fry for 5 mins. Add the garlic and squash, and cook for 5 mins more. Pour in the Madeira and stock, stir in the chopped sage, then cover and simmer for 20 mins until the squash is tender.
Blitz with a hand blender or in a food processor until completely smooth. Allow to cool in the pan, then chill until ready to serve. Will keep for 2 days or freeze for 3 months. To make the crispy sage, heat some oil (a depth of about 2cm) in a small pan, then drop in the sage leaves until they are crisp – you will need to do this in batches. Drain on kitchen paper. Will keep for several hours.
Just before serving, reheat the soup in a pan. The texture should be quite thick and velvety, but thin it with a little stock if it is too thick.
For the apple croutons, heat the oil in a large pan, add the apple and fry until starting to soften. Sprinkle with the sugar and stir until lightly caramelised.
To serve, ladle the soup into small bowls and top with the apple, sage and a grinding of black pepper.