Peel boiling onions and divide evenly between 3 clean pint sized Mason jars.
Combine water and canning salt. Stir until the salt is fully dissolved. Pour enough brine into each jar to cover the onions.
Cover the onions with a canning lid and seal with a canning ring. The onions need to sit in the brine for 24 hours prior to making them.
After 24 hours, drain onions and rinse well, leaving the onions in the jar.
Fill a water bath canning pot with enough water to cover the jars by about an inch. Cover the pot and bring the water to a boil.
In a small saucepan over medium-high heat, combine the cider vinegar, pickling spices and sugar. Bring mixture to a boil and allow to boil for two minutes. Remove from heat.
Pour the vinegar mixture evenly over onions, dividing between the three jars. Leave 1/4 inch headspace. Wipe the jar rims and seal the jars (fingertip tight).
Process 10 minutes in water bath canner then remove and allow to cool. Allow two weeks before eating for full flavor development.