Begin by thinly slicing the cucumbers and onion. You can use a mandoline or a sharp knife for this. Place the sliced cucumbers and onion in a colander and sprinkle them with pickling salt. Let them sit for about 1 hour to draw out excess moisture.
After an hour, rinse the cucumbers and onion under cold water to remove the salt. Make sure to thoroughly rinse them to get rid of any excess saltiness. Drain them well and set aside.
In a large saucepan, combine the white vinegar, granulated sugar, mustard seeds, turmeric, celery seeds, and crushed red pepper flakes. Bring the mixture to a boil over medium heat, stirring occasionally until the sugar is completely dissolved.
Add the sliced cucumbers and onion to the pickling liquid. Stir well to ensure all the vegetables are coated. Let the mixture simmer for about 5 minutes, or until the cucumbers have slightly softened.
Using a slotted spoon, transfer the pickles into sterilized glass jars. Pour the pickling liquid over the cucumbers, leaving about 1/2 inch of headspace at the top. Make sure the pickles are fully submerged in the liquid.
Seal the jars tightly with sterilized lids and let them cool to room temperature. Once cooled, refrigerate the jars for at least 24 hours to allow the flavours to meld together.