Chop the onion finely and cooked watercress, tarragon, wine and vinegar. Boil until the left 1/4 of the liquid - about 50-70 ml. Remove from heat and cool. Strain. Beat the egg yolks with salt. At constant beating with wire is added to the boiled liquid. Put it in a water bath and beat until it begins to thicken. Add the butter lump by lump, stirring rapidly until complete uptake of the butter and the preparation of creamy consistency. Depending on the size varies yolks and commitment butter. * The sauce belongs to classic French cuisine, best suited for Chateaubriand, stewed fish, light meats.