Step 1Trim the cauliflower into florets, discarding the core, large stems and leaves. Set aside.
Step 2Place the pickling spice and red pepper flakes in cheesecloth and tie to make a secure but loose bundle, or sachet. Set aside.
Step 3In a medium pot, combine the water, white and rice vinegars, sugar, salt, garlic and dill, along with the pickling spice sachet. Heat the mixture over medium-low heat, stirring occasionally until the sugar and salt dissolve. The mixture should not come to a simmer, but a light steam will begin to roll from the top of the liquid.
Step 4Remove from heat and set the liquid aside to cool for 5 minutes, then pour the warm liquid over the cauliflower florets. Place a weighted object, such as a small plate, over the florets to keep them submerged, then place the bowl in the refrigerator to chill for a minimum of 12 hours.
Step 5Store the cauliflower, chilled and submerged in brine, for up to 2 weeks. For a longer shelf life, use a traditional canning method.