Wash and trim the cucumbers; slice them into coins. Use a crinkle cutter if you wish. Slice the onions thinly.
Toss cucumbers and onions with canning salt in a big bowl. Cover the vegetables with ice and let them rest for 2 hours.
After about an hour and a half, sterilize jars, lids, ladle, canning funnel, and tongs. Fill the water bath and bring it to the boil.
Drain the vegetables in a colander, rinse well, and pat everything dry. Crisp is the goal.
Make the brine: bring vinegar, sugar, and your spices to a gentle boil. Stir until the sugar melts. Add the vegetables to the saucepan and bring it back to a boil.
Set a canning funnel on a hot, clean jar. Ladle the cucumber–onion mixture into the jars, leaving ½ inch headspace in each.
Slide a clean chopstick or plastic knife down the sides to release air bubbles. Adjust headspace if needed.
Wipe rims with a damp paper towel.
Add two-piece lids. Screw bands on fingertip-tight.
Process jars in a boiling-water bath for 10 minutes.
Lift jars out and place on a towel. Let cool, undisturbed, for 12–24 hours.