Whisk the whites in a pan with some water of the cold stock , vinegar and tarragon.
Heat the rest of the stock, add the gelatine (previously soaked for 20 min in cold water) and whisk till dissolved.
Add the stock and dissolved gelatine into a pan, whisk well.
Place on a stove and allow to come gently to the boil until clarified.
Strain through a muslin.
Repeat if needed using egg whites to give a crystal clear aspic.