Place all ingredients in a covered jar and allow to sit for a couple of days so that the seeds soften. Don’t get too persnickety about the time frame. I can tell you with absolute certainty that if your week gets a little out of control busy with too many people needing too many things, it will be OK if you leave the soaking seeds sitting on your counter for a full week.
Over the course of a couple of days, the seeds will absorb almost all of that liquid. If the liquid is completely absorbed and your seeds look a little dry, add a bit more apple cider vinegar. Mine didn’t need any more liquid, but yours might.
After a couple of days (or a week) pour the contents of the jar into your blender and process until the mustard is the desired consistency. Less if you like it grainy, more if you prefer it creamy. Put the finished mustard right back into the jar the seeds were inches Not even a jar to wash!
You can keep simple and just added a bit of thyme for seasoning or you can go crazy with different flavors. If you like traditional yellow mustard, add a bit of turmeric. You can use beer or white wine in place of the apple cider vinegar for a different flavor, too.
The mustard mellows a bit as it sits, but it’s still quite bold.