Step 1 Bring the balsamic vinegar to a boil, then remove from heat and pour the mustard seeds over the vinegar. Cover and set aside overnight. The next day, stir the simple syrup in with the seeds. If the liquid isn't completely absorbed, strain the seeds before using. The pickled seeds should have a caviar-like texture. This makes more pickled mustard seeds than is required for the remainder of the recipe; the seeds will keep, covered and refrigerated, up to 2 weeks.