Rinse and dry cucumbers. Slice very thin piece off flower end of each cucumber, if desired, to help prevent soft pickles.
Using food processor or mandolin, slice cucumbers into thin even slices crosswise.
Peel and slice onions into thin strips or rings. Combine with cucumbers in large pot. Sprinkle pickling salt over top and cover with ice, then very cold water. Refrigerate over night.
Drain very well. Add brine ingredients to cucumber mixture and bring to a boil.
Meanwhile prepare hot sterilized jars. Warm seals.
Once mixture has come to a rolling boil, fill each jar with cucumber mixture. Pour hot brine over top leaving at least ¼ inch headspace at top of each jar. Wipe rims to remove any sticky residue, then top with warm clean seals and rings.
Immerse jars in a large canner pot until covered and boil for 5-8 minutes, depending on size of jars.
Remove from water and allow to cool. Any jars that don't seal can be stored in the refrigerator. Pickles can be eaten right away.