Combine cucumbers, onions, and canning salt in a bowl. Cover with ice water and let sit for 2 hours.
Transfer mixture to a colander, rinse with cool water, and pat dry.
Sanitize jars, lids, and canner before making pickling liquid.
Boil vinegar, sugar, mustard seeds, turmeric, and celery seeds. Add cucumber and onion mix and bring to a boil.
Pack vegetables into jars, leaving ½ inch of space at the top.
Pour hot pickling liquid over veggies, remove air bubbles, adjust headspace if needed.
Wipe jar rims, seal with lids, and tighten bands.
Place jars in boiling water bath canner, process for 10 minutes.
Remove jars, let cool for 24 hours, check seals.
Store canned pickles in a cool, dark place for up to one year.