Wash the cucumbers well, trim off the blossom ends, and slice the cucumbers into even 1/4-inch rounds. Thinly slice the onions and place both in a large nonreactive bowl.

Sprinkle the salt over the cucumber and onion mixture, toss well, and cover with crushed ice. Let stand for 2 hours.

Prepare the water-bath canner, jars, lids, and bands according to current safe canning guidance. Keep jars hot until ready to fill.

Drain the cucumber mixture thoroughly and rinse well under cool water. Drain again very well.

In a large nonreactive pot, combine the white vinegar, apple cider vinegar, sugar, mustard seeds, celery seeds, turmeric, cloves, and allspice. Bring to a gentle boil over medium heat, stirring until the sugar dissolves.

Add the drained cucumbers and onions to the hot brine. Return just to a simmer and cook for 3 to 5 minutes, until heated through but still crisp-looking.

Using a slotted spoon and ladle, pack the hot cucumber and onion slices into hot pint jars, then pour in the brine to cover, leaving 1/2 inch headspace. Distribute spices among the jars.

Remove air bubbles, adjust headspace if needed, wipe the jar rims, and apply lids and bands to fingertip tight.

Process the pint jars in a boiling-water canner for 10 minutes, adjusting for altitude if needed.

Turn off the heat, wait 5 minutes, then remove the jars and place them on a towel-lined surface to cool undisturbed for 12 to 24 hours.

Check the seals, label the jars, and let the pickles rest for at least 1 week before opening for the best flavor.