Scrub the cucumbers well, and trim away the stem and blossom ends. Cut into 1/2 inch slices and toss with the onions in a large bowl.
Cover with a brine of ½ cup kosher salt dissolved into 1 gallon of cool water. (Don’t worry if some of the salt doesn’t dissolve immediately). Crack two trays of ice over the top and set aside for two hours. Stir from time to time, turning over the layers.
Meanwhile, combine the remaining ingredients in a saucepan. Bring to a boil, then remove from the heat and set aside.
Drain the vegetables, and rinse them with fresh water. Bring the vinegar syrup back to a boil, add the vegetables and cook for 5 minutes.
Using a slotted spoon or clean hands, divide the vegetables among four sterilized pint jars. Then ladle the syrup into the jars, leaving ½ inch head-space.
Seal with the lids and rings and process in a boiling water bath for 10 minutes.