Mustard Caviar

In a small saucepan, bring 1/2 cup rice vinegar, 1/4 cup brown mustard seeds, 1/4 cup yellow mustard seeds, 1/4 cup apple cider vinegar, 1/4 cup sugar, 1/2 tablespoon kosher salt, and 3/4 cup water to a boil over medium-high heat. Reduce heat to low and simmer, stirring often and adding water by tablespoonfuls if mixture is too dry, until mustard seeds are still firm and intact but pop like caviar when bitten into, 45-50 minutes. Remove saucepan from heat and let mustard mixture cool completely. Transfer to an airtight container and refrigerate. Keep chilled in the fridge for up to 2 weeks.