Combine ½ cup yellow mustard seeds, ¾ cup water, ¾ cup rice wine vinegar, ¼ cup sugar, and ½ tablespoon kosher salt in a small saucepan

Bring to a gentle simmer over low heat

Cook, stirring every few minutes, until seeds are plump and tender, about 45 minutes

If seeds start drying out, add a splash of water to keep them just barely submerged

Remove from heat and let cool to room temperature

Transfer to a jar or airtight container and store in the fridge — will keep for months