In a large bowl gently toss the cucumbers, onions, and kosher salt. Transfer to colander set in extra-large bowl, layering with ice, and finishing with a layer of ice. Weight with heavy plate. Chill overnight, up to 24 hours.
Meanwhile, for pickling syrup, in large nonreactive (stainless, enamel, or nonstick) saucepan combine sugar, vinegar, mustard seeds, celery seeds, and ginger. Bring to boiling; reduce heat and simmer, uncovered, 3 minutes. Remove from heat. Cool, cover, and refrigerate until ready to proceed with recipe.
After cucumbers have chilled, remove any unmelted ice and discard any liquid in bowl. Transfer cucumber mixture to nonreactive Dutch oven.
Strain syrup through a large sieve lined with cheesecloth over cucumbers. Bring mixture just to a low boil, stirring occasionally. Remove from heat.
With a large spoon transfer cucumbers to hot sterilized pint canning jars, leaving 1/2 inch headspace. Bring syrup in Dutch oven to boiling. Ladle hot syrup over pickles to cover. Wipe jar rims with damp cloth. Put on lids and screw bands. To seal, invert jars until cool. Store in refrigerator. Makes 5 pints (40 1/4-cup servings).