Peel the shallots, cut the larger ones in half. Place all the ingredients up to and including the salt in a pan and bring to the boil, reduce the heat, cover and cook over a medium heat for approx. 15 mins. Remove the shallots, transfer to 2 sterilized jars (approx. 500 ml each).
Add the balsamic, sugar and salt to the liquid, return to the boil, pour the boiling hot liquid over the shallots to cover. Seal the jars immediately, allow to cool on a towel.