Cut a thin slice from the ends of each cucumber and cut into thick slices. Place cucumbers, califlower and onions in a large non-reactive container. Combine salt with lukewarm water, stirring until dissolved. Pour over vegetables and let stand 24 hours. Make sure vegetables are covered, I often have to make the brine a couple of times to have them covered.
After 24 hours, drain the vegetables well, and rinse them well.
Combine sugar, flour, mustard, celery seeds and tumeric in a large saucepan, stir until well mixed. Whisk in vinegar and water. Bring to to a boil over high heat stirring constantly, until smooth and thickened. Add vegetables and return to a boil for 30 seconds.
Remove hot jars from canner. Remove vegetables from liquid with a slotted spoon; pack into jars. Process 10 minutes for pints (500 ml) jars and 15 minutes for quart (1 L) jars.