In a small saucepan, bring all ingredients plus 3/4 cup water to a boil over medium-high heat.
Reduce heat to low and simmer, stirring often and adding water by the tablespoon if mixture is too dry, until mustard seeds are still firm and intact but pop like caviar when bitten into, 45–50 minutes.
Remove from heat and let cool completely. Transfer to an airtight container and refrigerate. Keep chilled up to 2 weeks.