Leek cappuccino soup

Method

1. Put the stock, milk, bay leaves and thyme in a saucepan over a high heat, then bring just to the boil. Reduce the heat to low to keep warm.
2. Meanwhile, melt the butter over a low heat in a large saucepan. Add the leeks, potato and a pinch of salt, stir to coat, then turn up the heat to high and cook for 10-12 minutes until tender, stirring frequently to ensure the vegetables don’t colour.
3. Pour the stock and milk mixture over the vegetables. Simmer for
   15 minutes, then take off the heat and discard the herbs. Using a stick blender, blend until smooth. Pass through a sieve into a deep saucepan (with enough room to submerge the blender blades for frothing later). The soup should have a velvety texture (add a little stock or water if you need to). Taste, season and keep in the saucepan until ready to serve (see Make Ahead).
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5. To serve, gently warm the soup, then fill each cup three-quarters full. Whizz the remaining soup with the stick blender until frothy, then spoon into the cups.