For the stock, heat the chicken stock, onion, carrot and celery in a saucepan until boiling. Reduce the heat, add the pork belly and simmer for one hour. Strain the mixture into a jug and set aside. Discard the vegetables.
Remove the meat from the pork, cut into small pieces and set aside.
For the soup, heat the butter in a saucepan until foaming. Add the reserved pork, shallots, carrots, celery and leek and fry for 2-3 minutes. Pour in 750ml/1pint 5fl oz of the reserved stock and 150ml/5floz of the double cream and bring to a simmer. Season, to taste, with salt and freshly ground black pepper.
Meanwhile, whisk the remaining 100ml/4fl oz double cream with the egg yolk in a bowl until well combined. Whisk the mixture into the soup and simmer for 1-2 minutes.
To serve, ladle the soup into 4 cups and garnish each cup with a spoonful of cream, some chives and croutons.
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James Martin presents, with guest chefs Vivek Singh and Hélène Darroze.