Place the mustard seeds and coriander seeds in a small saucepan over medium heat. As soon as the mustard seeds start to jump turn off the heat and transfer to a mortar & pestle, spice grinder or even a blender. If you do not have any of these devices, simply use the powdered mustard, ground coriander and skip the pre-toast.
Add the orange juice, apple cider vinegar, honey and salt into the saucepan. Bring it up to a simmer for just a few minutes. Toss the spices into the pan.
Slice off the leaves of the cauliflower and pull apart into florets. Place the florets in a jar with a lid or shallow dish with a lid.
Pour the steamy liquid over the top. Seal the jar or dish. Allow to rest at room temperature, shaking periodically. Give the cauliflower at least an hour before serving.
They will store nicely in their liquid for about a week- if they are not devoured prior. If the flavors become too intense, drain off the liquid.