Combine cucumbers, onions, and salt in a large non-reactive bowl (glass or stainless steel). Mix until cucumbers and onion are fully coated. Let sit for two hours.
After two hours, drain the cucumber/onion mixture and rinse well to remove most of the salt.
In a large stainless steel pot, combine remaining ingredients and bring to a boil, ensuring sugar has dissolved. Add cucumbers and onions to the pot and return to a boil.
Remove from heat and start filling jars. Using a slotted spoon, scoop solids equally into five pint-size mason jars. Top with brine, bringing contents up to ½ inch headspace. De-bubble the jars with a wooden chopstick or plastic utensil. Top up with brine to return to ½ inch headspace. If you are short on brine, you can add white vinegar.
Wipe jar rims with a paper towel wetted with vinegar, then put on two-part lids and screw to fingertip tightness. Place in water bath canner and cover with hot water until jars are submerged by 1-2 inches of water. Process in canner for ten minutes; once the water starts boiling, start the ten minute timer. If at any point the water stops boiling, return to a boil and restart the ten minute timer.
When processing time is over, turn off heat and let canner sit for five minutes. Then, carefully using a jar lifter, remove jars from canner and place on a thick towel on a counter or table and let them sit undisturbed overnight.
In the morning, check to see that lids have sealed (they dip inward). If so, they can be stored on a shelf for up to a year; after which safety does not change, but quality may degrade. If any jars did not plea (they click when you press on them), store these jars in the fridge and consume them first.
Notes
If using quart jars, add 5 minutes to processing time.