Use the large holes of a box grater to grate cucumbers and onions into a large bowl. Sprinkle pickling salt over grated vegetables. Cover the bowl tightly; allow to rest at room temperature for 8 hours to overnight.

Squeeze out as much liquid as possible from cucumber mixture; set aside.

Whisk together sugar, flour, turmeric, ginger, and celery seed in a second large bowl. Heat vinegar and water in a large, heavy pot over medium-high heat. Whisk in sugar mixture until smooth. Stir in cucumber mixture. Bring to a boil and cook, stirring frequently, until relish is thickened, about 15 minutes.

Sterilize the jars and lids in boiling water for at least 5 minutes. Pack 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 the lids; screw on the 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 water level is at least 1 inch above the tops of the jars. Bring water to a full boil, cover the pot, and process the jars for 10 minutes.

Remove the jars from the stockpot and place them 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 (the lid does not move up or down at all). Store in a cool, dark area.