Grate the cucumbers and onions into a large bowl using the large holes of a box grater. Sprinkle the pickling salt over the top. Cover tightly, and allow to rest at room temperature overnight.
Squeeze as much liquid as possible out of the cucumber mixture; set aside. Whisk together the sugar, flour, turmeric, ginger, and celery seed. Heat the vinegar and water in a large, heavy pot over medium-high heat. Whisk in the sugar mixture until smooth. Stir in the cucumber mixture. Bring to a boil and cook, stirring frequently, until thickened, about 15 minutes.
Sterilize the jars and lids in boiling water for at least 5 minutes. Pack the relish into the hot, sterilized jars, filling the jars to within 1/4 inch of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil over high heat, then carefully lower the jars into the pot using a holder. Leave a 2 inch space between the jars. Pour in more boiling water if necessary until the water level is at least 1 inch above the tops of the jars. Bring the water to a full boil, cover the pot, and process pint jars for 10 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area.
