Using a chef's knife or mandoline, slice the cucumbers evenly, discarding the ends, and place in a large bowl along with the yellow onion and salt. Cover the ingredients with cold water. Cover the bowl with plastic wrap and allow to sit at room temperature for 2-3 hours. Drain the cucumbers and onions in a colander, rinsing thoroughly.
In a dutch oven or large saucepan, combine the vinegar, sugar, mustard seed, turmeric and celery seed. Bring to a boil, add the cucumbers and onions, and cook for 2 minutes, stirring.
Remove from the heat, place in airtight jars and store in the refrigerator for up to 2 weeks. It will likely take 12-24 hours to fully develop their flavor.