In a large bowl, toss cucumbers, onions, and salt.
Transfer mixture to a large colander, alternating layers with crushed ice, finishing with ice on top.
Weight with a heavy plate and chill overnight, up to 24 hours.
Meanwhile, in a large nonreactive saucepan, combine sugar, vinegar, mustard seeds, celery seeds, and ginger; bring to a boil, then reduce heat and simmer, uncovered, 3 minutes.
Remove from heat, cool, cover, and refrigerate until cucumber mixture is ready.
Remove any unmelted ice from cucumbers and transfer to a nonreactive Dutch oven.
Strain liquid through a sieve, lined with cheesecloth, over cucumbers.
Heat cucumbers to a low boil, stirring occasionally; remove from heat.
Spoon cucumbers into sterilized pint sized canning jars.
Pour syrup over cucumbers to cover, leaving 1/2 inch head space.
Wipe jar rims with a damp cloth, put on lids and bands.
Invert jars, until cooled, to seal; store in refrigerator.