Rinse pickling cucumbers, cut the ends, and slice into ¼ inch rounds. Combine with onions and pickling salt in a bowl, cover with ice, and refrigerate for 4 hours.

Drain excess liquid from cucumbers and rinse thoroughly under cold water. Drain again to ensure mostly dry.

Prepare canning jars by submerging them in hot water for about 10 minutes.

Combine white vinegar, apple cider vinegar, sugar, and pickling spices in a pot. Heat until boiling and let simmer until sugar dissolves.

Add drained cucumbers and onions to the brine, stir gently and boil for 2-3 minutes.

Carefully fill prepared jars with the cucumber and onion mixture, pour hot brine over, leaving ½ inch headspace.

Wipe jar rims, secure lids, and process in boiling water for 15 minutes for proper sealing.

Remove jars from water and let cool on a towel. If not canning, refrigerate immediately.