Make the pickled mustard seeds: Rinse mustard seeds in a fine mesh strainer, pat dry with paper towels, and place in an 8-ounce glass jar or heatproof bowl. In a small saucepan, combine vinegar, sugar, chile, and salt. Bring mixture to a boil over medium heat, stirring to dissolve the sugar. Pour hot liquid over mustard seeds and let soak at room temperature until softened, about 3 to 4 hours. Store pickled mustard seeds in a sealed jar in refrigerator for up to 4 months.