In a large bowl, combine the sliced cucumbers, onion, and ½ cup salt. Mix to incorporate and pour into a large colander. Set inside a large bowl for 3 hours to drain excess water.

After 3 hours, discard the drained liquid. Rinse the cucumbers and onion well and allow to drain.

Prepare the water bath, 8 pint jars, lids, and rings.

In a 4-quart kettle, combine the vinegar, water, sugar, turmeric, cloves, celery seed, mustard seed, red pepper flakes, and 2 tsp salt. Stir to dissolve the sugar and bring to a boil. Reduce to a simmer while preparing the cucumbers in jars.

Pack jars with rinsed and drained cucumbers and onions, leaving ½-inch headspace. Pour brine over prepared cucumbers and onions, still leaving that ½-inch headspace. Wipe the rims of the jars with a clean cloth to remove any drips. Place on lids and rings, tightening to fingertip tightness.

Place jars in the boiling water bath. Bring back to a boil and boil for 10 minutes. Check with the USDA canning guide and adjust the process time for altitude.

Remove from the water bath and cool for 12-24 hours. Remove rings to ensure a good seal and store for up to one year.