Combine the vinegar, wine, onion and half of the tarragon in a small pan. Cook until most of the liquid has evaporated leaving you with about 15 ml. Remove from the heat. Whisk the egg yolks and the water together. Add to the pan. Whisk until pale and thick. Return to very low heat and continue to whisk vigorously reaching all over and inside the pan with the whisk the until egg start to cook and thicken. Remove from the heat every now and again. Remove from the heat and whisk in the butter – tablespoon by tablespoon - until a thick emulsion is formed. Return briefly to the heat to warm up while whisking continuously. Season with pepper and a few drops of lemon juice. Stir in the remaining tarragon.