Take a large pot or bowl.
Place the cucumber, onion and cauliflower into a pot.
Add the vinegar, sugar, mustard, turmeric, celery seeds and salt.
Simmer gently until thickened, about 20 minutes, stir frequently.
Now turn off the heat. In a small bow, combine the cornstarch with about 2 tablespoons water to make a slurry, then whisk the slurry into the vinegar mixture.
In a small bowl, combine the all-purpose flour with about 3 tablespoons water to make a slurry, and then whisk the slurry into the vinegar mixture.
While the mixture is hot, pour into the Mason jars.
The filling should be within 1/2 inch of the top. cap them.
After about 15 minutes, test the tops with a gentle push to make sure they are sucked in.
Store your pickles in a cool, dry, dark place. There is no need to refrigerate until opened