Wash the cucumbers and trim the tops and ends. Slice the cukes into rounds about 1/4-inch thick or a bit less.
Quarter the onions and slice them about 1/4 inch thick.
layer the cucumber slices and onions with the pickling salt in a large bowl. Cover with about 6 cups of ice cubes. Let the mixture stand for 3 to 4 hours.
Meanwhile, put the jars in a large canner and cover them with water. Bring to a boil. Reduce heat to the lowest temperature to keep the jars hot until ready to fill them.
Put the lids in a saucepan and bring them to a simmer. Keep them warm until you’re ready to seal the jars.
Put the cucumber and onion mixture into a large colander (or two colanders) and rinse thoroughly under cold running water.
combine the sugar, cider vinegar, white vinegar, mustard, celery seeds, and turmeric in a large stainless steel pan. Slowly bring the vinegar mixture to a full boil over medium heat, stirring frequently to dissolve the sugar. Stir in the cucumber mixture, increase the heat to medium-high, and return to a full boil.
With tongs, remove a jar from the hot water in the canner and fill it with the cucumber mixture, leaving a 1/2-inch head space. With dampened paper towels, wipe the rims of the jar. Center a lid on a jar so the sealing compound is in contact with the rims. Screw the band down to fingertip tightness. Do not over-tighten. Repeat until all of the pickles are used.
Put the filled jars in the canner If the water isn’t at least 1 inch above the tops of the jars, add more hot or simmering water. Make sure the jars do not touch each other. Cover the canner and bring to a full boil. Reduce the heat and keep the water at a gentle boil for 10 minutes. Turn off heat, remove the cover, and let stand for 5 minutes.
With tongs, remove the jars from the water to a rack or heavy towel to cool.
Do not tilt or disturb lids while they cool.
After 24 hours, check the seals and remove the bands.
Label and store in a cool, dry, dark place.
For the best flavor, wait at least 2 weeks before opening the first jar.