Place onions in medium heatproof bowl. Add boiling water to cover. Let stand 15 minutes. Drain and rinse with cold water.
Meanwhile, bring remaining ingredients to simmer in small saucepan on medium-high heat. Remove from heat. Let stand 15 minutes. Pour over onions in medium bowl. Cover.
Refrigerate 2 to 3 hours to marinate onions. Drain before serving. Store in covered glass or ceramic container in refrigerator up to 1 week.