Gather the tarragon herb just before it flowers, and on a dry day before the sun has shone on it. Wash it, shake it out of doors, and dry it on a clean cloth. Pick off the leaves, place them in a wide-necked bottle, fill the bottle with the best vinegar, and let it stand for two weeks. Strain through muslin into another bottle, stand uncorked in a saucepan of water reaching almost to the neck, bring the water to the boil, and let it boil for five minutes.
Cork and set aside for use.
Similar vinegar can be made from green mint, horseradish root, garlic, shallots, onions and cucumbers.