Soak the mustard seeds in the vinegar and beer or water for two days. Add the sugar and salt and purée; the brown mustard seeds will remain whole. If the mustard is too thin, add some ground yellow mustard to thicken it. Refrigerate. The mustard will mellow a bit in the refrigerator for a day or so.
¼ cup brown mustard seeds (optional)
¾ cup apple cider vinegar
¼ cup plus 2 tablespoons beer or water
1½ teaspoon granulated sugar
1½ teaspoon kosher or sea salt
ground yellow mustard as necessary
Some of us might prefer more beer and less vinegar. I say go ahead and reverse the quantities, or devise your own ratio. Leave out the brown mustard seeds if you don't want seeds in your mustard. If you want a honey mustard, leave out the sugar, make the mustard, then mix with as much honey as you wish, typically equal parts honey and mustard.
Some of us might want garlic mustard, or tarragon mustard, or some other mustard-y variant. Great! Add whatever you like.
If you think making a cup or so of mustard in one batch is more than you're going to use, cut the quantities in half. Or just make the whole batch, and then use it to make two or three smaller mustard variants.