Combine ingredients in a nonreactive mixing bowl. Cover with plastic wrap and let sit at room temperature for 1–2 days so that the mustard seeds soften and the flavors meld.
Transfer the mixture to the bowl of a food processor and process, stopping occasionally to scrape down the sides of the bowl with a rubber spatula, until the seeds are coarsely ground and the mixture thickens, about 3 minutes. Transfer to a jar and cover. Keeps in the refrigerator for up to 6 months.