How to cook
Chop the onion finely and put it in a pot to simmer with the vinegar and wine over medium heat. When the vinegar boils off completely, remove the pot from the heat and allow it to cool.
In a water bath, beat the butter with the salt, black pepper and tarragon and gradually add the egg yolks. To this mixture, start adding the cooled mixture and vinegar, beating constantly with a whisk, the mixture must thicken and begin to stick to the spoon.