Make the brine by combining the white vinegar, apple cider vinegar, water, salt and sugar in a saucepan. Bring to a simmer, stirring occasionally to dissolve the salt and sugar.
Meanwhile, slice the cucumbers into round 1/8-1/4 inch slices. A crinkle cut slicer is fun and traditional but isn’t required.
Layer the cucumber slices into a quart jar, layering with the onion slices. Then add the mustard seeds, coriander seeds, celery seed (if using) and turmeric.
Pour the warm vinegar brine over the cucumbers, making sure to cover them completely.
Cap the jar with a lid and place in the refrigerator. Wait 2-3 days before eating to let the flavor develop. They will keep for 2-3 months in the refrigerator.
Notes
Any type of cucumber can be used, but pickling cucumbers are the least likely to go mushy. I’ve had good luck with many different varieties!
Many recipes call for salting the sliced cucumbers and letting them sit for several hours before brining. I don’t find this to be necessary and can be skipped, but it is an optional step if you prefer.