* 1 small Onion, finely chopped
* 90ml White wine vinegar
* 250g Unsalted butter, chilled and chopped into small pieces

===Method===
* Cook the Onion in the Vinegar in a small pan until there are only 4 tablespoons mixture remaining.
* Strain and discard the Onion.
* Return the Vinegar to the pan.
* With the Vinegar on a very, vey low heat, gradually add the small pieces of Butter and whisk constantly.
* The Butter should make a thick Sauce.
* '''Classic French Cooking''' by ''Elisabeth Luard'' (ISBN 1840727241)

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