Wash and slice: Rinse cucumbers well, trim off the blossom ends, then slice into thin rounds. Thinly slice the onions and place everything in a large bowl.
Salt and chill: Sprinkle pickling salt over the cucumbers and onions. Cover with ice and refrigerate for 3–4 hours, then drain well.
Prepare jars: Wash jars, lids, and rings. Keep jars hot and prepare the water bath canner before filling.
Make the brine: Combine vinegar, sugar, mustard seed, celery seed, and turmeric in a large pot. Bring to a boil and boil for 10 minutes.
Add cucumbers: Add the drained cucumber and onion mixture to the hot brine. Slowly heat back to boiling.
Fill and process: Pack hot pickles into hot jars, cover with brine, and leave 1/2-inch headspace. Remove bubbles, wipe rims, apply lids, and process in a boiling water canner for 10–20 minutes based on altitude.
Cool and store: Let jars cool for 12–24 hours. Check seals, label jars, and store in a cool, dark place. For best flavor, wait 4–5 weeks before opening.
