Combine white wine vinegar, cider vinegar, water, whiskey, bay leaves, brown sugar, and salt in a small saucepan.
Set aside, stirring occasionally, until sugar is dissolved.
For the Seeds:
Place seeds in a clean saucepan and cover with cold water.
Bring to a boil, then drain through a fine-mesh strainer and discard the water.
Repeat until the discarded water does not taste bitter.
This can take between 1 and 8 repetitions.
Combine and Complete
Transfer blanched and drained mustard seeds to the pot with the brine.
Bring to a boil, then reduce to a bare simmer and cook until the seeds have plumped up and the liquid has reduced by about one-third, 15 to 20 minutes.
Without draining brine, transfer seeds to a sealable container, let cool at room temperature.
Then seal and refrigerate until ready to use.
The seeds will mature in flavor over the course of a few days.