Step 1: In a large non-reactive bowl, combine the thinly sliced cucumbers and onions. Sprinkle with pickling salt and toss gently to coat. Let stand at room temperature for 2-4 hours, or refrigerate overnight, to draw out excess water.
Step 2: Drain the cucumber and onion mixture thoroughly in a colander. Rinse under cold running water for a minute or two to remove excess salt, then drain very well again. Gently squeeze out any remaining water without crushing the vegetables.
Step 3: In a large non-reactive pot, combine the apple cider vinegar, white vinegar, granulated sugar, brown sugar, mustard seeds, celery seeds, ground turmeric, ground ginger, and optional red pepper flakes. Bring the mixture to a boil over medium-high heat, stirring constantly until all sugars are dissolved.
Step 4: Reduce the heat to a simmer and add the drained cucumber and onion slices to the pot. Stir gently to ensure all vegetables are submerged. Bring the mixture back to a gentle simmer, but do not boil aggressively. Cook for about 5-7 minutes, until the vegetables are heated through and just begin to turn translucent.
Step 5: Carefully ladle the hot pickles and brine into sterilized pint or quart canning jars, leaving about 1/2-inch headspace. Make sure the vegetables are fully covered by the brine. Remove any air bubbles. Wipe the rims clean, center the hot lids, and apply the bands finger-tight.
Step 6: (Optional for longer storage) Process the jars in a boiling water bath canner for 10 minutes. Remove jars and let cool completely on a wire rack for 12-24 hours. Check seals; any unsealed jars should be refrigerated. For best flavor, let the pickles rest for at least 2 weeks before opening. Store sealed jars in a cool, dark place, and refrigerated once opened.