Put Pasolivo Tuscan Balsamic Vinegar in a small saucepan, bring to boil, and then remove from heat. Stir in Pasolivo Kosher Flake Sea Salt to dissolve and add peppercorns. Add the sliced onions to the jar, then pour the hot vinegar mixture into the jar. Let cool to room temperature, cover with lid, and refrigerate. Use within two weeks.