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. If you want to speed up the cooling process, carefully transfer hot brine to a heat-proof jar or bowl and place in fridge or freezer to cool it down faster.

Prepare jars: While brine is cooling, wash two wide-mouth 1-pint jars and their lids in very hot, soapy water. Set aside.

Prepare cucumbers, onion, red pepper: Wash and dry the vegetables. Cut off the ends of the cucumbers and discard. Then slice cucumbers into coins, aiming for ⅛" thickness - a mandoline works great for even slicing. Or slice cucumbers into spears. 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. Then add the cucumbers, onion, and red pepper to both jars. Pack the veggies in as tightly as you can without smashing the cucumbers, layering them into the jars to distribute evenly. Then gently but firmly tap the bottom of the jars on a wooden cutting board or thick kitchen towel to settle everything into the jars as solidly as possible. Add more cucumbers if needed to leave ½" of space at the top of the jars. If using spears, trim the ends if needed, to leave ½" of space at the top.

Add pickling brine: Once brine has cooled to room temperature, pour it into the jars, filling each jar within ½" of the top. Gently tap the jars against a wood cutting board or thick towel again, to remove air bubbles. Top off with additional brine to cover the cucumbers. Twist lids tightly onto jars. Then turn jars upside down and shake to distribute spices. Turn jars right-side up and gently tap again on a cutting board to settle cucumbers solidly into the brine.

Store in refrigerator: These are quick refrigerator pickles that need to be stored in the refrigerator. Do NOT store these in the pantry. The flavor will increase the longer they sit, and will be very good after 24 hours, even better after 48 hours. If you're planning to serve these when entertaining, I recommend making the pickles 48-72 hours in advance, for the best flavor. They will keep well for up to 2 months.