Melt the butter in a large saucepan over a medium-low heat and sweat the leeks with the thyme, stirring from time to time, until soft – about 10 minutes.
Turn the heat up to medium-high and add the mushrooms with a pinch of salt (this will help the juices to run). Sauté for a few minutes until they soften and lose some of their moisture.
Add the garlic and stir for a minute, then sprinkle over the flour and stir for a couple of minutes. Pour over the hot stock. Bring to the boil and simmer gently, uncovered, for 20 minutes. Remove the thyme. Whiz the soup in a blender until smooth (or blend three-quarters of it and leave a quarter unblended if you prefer a soup with more texture). Return it to the pan.
Add the cream to the soup, along with the nutmeg, and reheat gently, stirring. Add the sherry, if using, then taste and adjust the seasoning, adding more salt, pepper and/or nutmeg as needed.
Spoon the soup into warmed bowls and add a swirl of cream and a sprinkling of chopped herbs before serving.
to finish
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