Slice the cucumbers into ¼-inch slices using a mandolin or cutting by hand.
In a large bowl, combine the cucumbers and salt. Put in the fridge for one hour then rinse well in a colander. Drain them in the sink for a few minutes then transfer them to a large bowl. Slice the onion and add it to the bowl with the cucumbers.
In a medium pot, combine the sugar, white vinegar, apple cider vinegar, mustard seeds, celery seeds, and turmeric. Cook on medium heat for 5 minutes.
Fill two 32-ounce jars with the cucumber onion mixture. Then pour in the brine. Let the jars cool for about 1 hour, then put the lids on and put in the refrigerator for at least 24 hours before eating.