In an extra large bowl, sprinkle salt over the top of prepared cucumbers and onions. Stir and place in the refridgerator overnight.
Strain and rinse the vegetables with fresh water very well to remove any excess salt.
Place 4 clean 500ml mason jars on a rack in a water bath canner and cover jars with water. Heat jars to a simmer (180°F/82°C). Place the lids and rings in a medium sized pot, fill will water and heat to a simmer (180°F/82°C). Keep both jars, lids, and rings hot till you are ready to use them.
In a 5 or 6-quart pot, stir together the white sugar and flour.
Add the vinegar, turmeric, mustard powder, and celery seed to the flour mixture and whisk together to create a smooth paste.
Add the rinsed vegetables to the smooth paste and fill with enough cold water to almost cover. Bring mixture to a boil over medium heat and stiring often.
Continue cooking for 2 hours or until desired thickness, stirring often.
Pour pickles into hot sterilized jars leaving a 1/2 inch of headspace. Seal with lids and rings. Process in boiling hot water bath canner for 15 minutes. Let stand in the canner for 5 minutes with lid removed. Be sure to refrigerate any jars that fail to seal properly.