Prepare Cucumbers and Onion:
In a large bowl, combine the sliced cucumbers, onion, and kosher salt. Toss well.
Let sit for 1 hour, then drain and rinse well with cold water.
Make the Brine:
In a medium saucepan, combine both vinegars, sugar, mustard seeds, celery seeds, turmeric, and peppercorns.
Bring to a boil over medium heat, stirring until the sugar is dissolved.
Combine and Pack:
Place the cucumbers and onions into clean glass jars (pint or quart size).
Pour the hot brine over the vegetables, ensuring they are fully submerged.
Cool and Store:
Let the jars cool to room temperature.
Cover with lids and refrigerate for at least 24 hours before eating for best flavor.