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: Wash and dry the cucumbers. Cut off the ends of the cucumbers and discard. Then slice cucumbers into coins, aiming for about ⅛" thickness - a mandoline works great for even slicing. Or slice them into spears. Then slice the onion about ⅛" thick as well.

Pack jars: Divide the celery seeds and mustard seeds between the two jars. Then add the cucumbers and onion to both jars. Pack them in as tightly as you can without smashing the cucumbers. 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 over cucumbers, 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, and keep well for up to 2 months. Do NOT store these in the pantry.