Prepare Pickling Brine
Place apple cider vinegar, white vinegar, water, sugar, salt, and turmeric in a small saucepan over high heat and bring to a boil. Whisk rapidly until sugar and salt dissolve. Remove pan from heat and let cool to room temperature. To speed up cooling, transfer hot brine to a heat-proof jar or bowl and place in fridge or freezer.
Prepare Jars
While brine is cooling, wash two wide-mouth 1-pint jars and their lids in very hot, soapy water. Set aside.
Prepare Vegetables
Wash and dry the vegetables. Cut off the ends of the cucumbers and discard. Slice cucumbers into coins (about 1/8" thickness) or spears. A mandoline works great for even slicing. Dice the onion and red bell pepper.
Pack Jars
Divide the celery seeds, mustard seeds, black peppercorns, and red pepper flakes evenly between the two jars. Add the cucumbers, onion, and red pepper to both jars. Pack tightly without smashing. Tap the bottom of the jars on a cutting board to settle everything. Leave 1/2" of space at the top.
Add Pickling Brine
Once brine has cooled to room temperature, pour it into the jars, filling each jar within 1/2" of the top. Tap jars again to remove air bubbles. Top off with additional brine. Twist lids tightly, turn jars upside down and shake to distribute spices.
Store & Wait
Store in refrigerator. Flavor improves after 24 hours, even better after 48 hours. For best flavor, make 48-72 hours in advance. Keeps for up to 2 months.