Place the vinegar and Pickling Spice in a saucepan. Bring to the boil. Leave to Cool for at least 2 hours, or overnight if possible. Strain.
Place the onions in a bowl. Mix together the salt and water. Pour over the onions. Leave for 24 hours. Drain and rinse. Pack tightly into clean jars. Pour over Spiced Vinegar. Seal jars.
Best eaten after 2 weeks. Use within 6 months.