Wash the pickling cucumbers thoroughly and slice into ¼-inch rounds. Trim off the blossom end as it contains enzymes that can soften pickles.

Combine sliced cucumbers, onion, and jalapeño peppers in a large bowl. Sprinkle kosher salt over the top and gently toss to coat all pieces evenly.

Cover the bowl with plastic wrap and refrigerate for 2-4 hours, stirring occasionally. This salting process draws out excess moisture for crispier pickles.

After salting time is complete, rinse the cucumber mixture thoroughly in a colander under cold running water until all salt is washed away (about 3-4 minutes). Drain well.

In a large pot, combine white vinegar, apple cider vinegar, sugar, mustard seeds, celery seeds, turmeric, peppercorns, garlic, and optional red pepper flakes. Bring to a boil over medium-high heat, stirring until sugar completely dissolves.

Add the drained cucumber mixture to the boiling liquid. Return to a simmer, then remove from heat. The cucumbers should change color slightly but still maintain their crispness.

Using a slotted spoon, transfer the cucumber mixture to sterilized jars, then ladle the hot pickling liquid over them, leaving ¼-inch headspace.

Seal jars and allow to cool to room temperature before refrigerating.

Refrigerate for at least 24 hours before enjoying to allow flavors to develop fully.