Step 1: Prepare Cucumbers and Onions. Wash cucumbers thoroughly. Using a mandoline or sharp knife, slice cucumbers into uniform 1/4-inch rounds. Peel and thinly slice the sweet onions.
Step 2: Brine the Vegetables. In a large non-reactive bowl (glass or ceramic), combine the sliced cucumbers and onions. Sprinkle with pickling salt and toss to coat evenly. Cover the bowl with plastic wrap and let it sit at room temperature for at least 2 hours, or up to 3 hours. This process draws out excess water, ensuring crisp pickles.
Step 3: Rinse and Drain. After brining, drain the cucumber and onion mixture in a colander. Rinse thoroughly under cold running water for several minutes, tossing gently to remove all excess salt. Taste a small piece to ensure it's not too salty; if it is, rinse again. Drain very well, pressing lightly to remove as much water as possible. Set aside.
Step 4: Prepare the Brine. In a large non-reactive pot or Dutch oven, combine apple cider vinegar, white vinegar, granulated sugar, light brown sugar, mustard seeds, celery seeds, ground turmeric, red pepper flakes (if using), and whole black peppercorns. Bring the mixture to a simmer over medium heat, stirring constantly until both sugars are completely dissolved. Do not boil vigorously; just a gentle simmer.
Step 5: Combine and Heat. Add the rinsed and drained cucumber and onion mixture to the simmering brine. Stir gently to ensure all vegetables are submerged. Continue to cook for about 5-7 minutes, just until the cucumbers turn a slightly translucent green and are heated through, but still retain their crunch. Do not overcook, or your pickles will be soft.
Step 6: Pack into Jars. While the pickles are simmering, sterilize your pint or quart canning jars and lids according to manufacturer's instructions. Using a slotted spoon, carefully transfer the hot cucumber and onion mixture into the hot, sterilized jars, leaving about 1/2-inch headspace.
Step 7: Ladle Brine and Seal. Ladle the hot pickling brine over the vegetables in each jar, ensuring the vegetables are fully submerged and maintaining the 1/2-inch headspace. Remove any air bubbles by gently tapping the jars or using a non-metallic utensil. Wipe the rims of the jars clean with a damp cloth. Center the lids on the jars and apply the bands, tightening them finger-tight.
Step 8: Water Bath Canning (Optional but Recommended for Shelf Stability). For shelf-stable pickles, process the jars in a boiling water canner for 10 minutes. Ensure the jars are covered by at least 1 inch of water. Once processing is complete, turn off the heat, remove the canner lid, and let the jars sit in the hot water for 5 minutes before carefully removing them to a wire rack to cool completely for 12-24 hours. Check seals; if any jar doesn't seal, refrigerate and consume within 2-3 weeks.
Step 9: Cool and Store. Let the sealed jars cool undisturbed on a counter for 12-24 hours. You should hear the lids pop as they seal. Once cool, check that the lids are concave and don't flex when pressed. Store properly sealed jars in a cool, dark place for at least 2 weeks before opening to allow flavors to develop. Refrigerate after opening. If not water bath canning, refrigerate immediately and consume within 2-3 weeks.