Combine ingredients in a bowl. Cover with plastic wrap and let rest at room temperature for at least 12 hours or up to 2 days. (This allows the mustard seeds to soften; at the end of the soaking process, most of the liquid will be absorbed.)
Transfer mixture to a food processor or blender. Process until the seeds are coarsely ground and the mustard has thickened, 4-5 minutes. Transfer to a jar. Mustard will keep in the refrigerator for up to 6 months.
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