Add both types of mustard seeds to your spice grinder, pulse three times (with a half-second pulse), then check the seeds. You're looking for most of the seeds to be cracked in half, although there may be a good amount of the smaller brown seeds that are still whole, which is okay. If you're not quite there, pulse again and recheck.
Transfer the cracked mustard seeds to the non-reactive bowl, along with all of the mustard powder.
Pour in the cool water and stir to mix thoroughly.
After 10 minutes, add the vinegar to stop the reaction and set the heat.
Stir in the salt and sweetener (if using).
Cover the bowl, place it in the refrigerator, and let it sit for 24 hours.
Notes
The 24-hour waiting period serves two important purposes: It lets the seasonings fully meld with the mustard, and it thickens everything nicely. Success!
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