Melt the butter in a saucepan over low heat, add the shallot and cook gently, without colouring, until soft and transparent.
Pour in the wine, scraping the base of the pan with a wooden spoon to lift and blend in the flavourful sediment. Stir in the stock, bring to the boil and then reduce the heat to a simmer. Boil, uncovered, until the liquid has reduced by h