How to Make It
In a medium nonreactive saucepan, combine the vinegar, sugar, cloves, allspice, bay leaves, peppercorns and mustard seeds and bring to a boil. Cook over moderate heat, stirring until the sugar dissolves, about 5 minutes. Remove from the heat and let cool completely, about 30 minutes.
Layer the onion rings in a 2-cup jar or plastic container. Strain the brine into the jar and let stand at room temperature for 1 hour. Drain the onions and serve.