1. Toss together first 5 ingredients in a large bowl. Cover and let stand 1 hour. Transfer to a colander; drain 20 minutes. Pack into 3 (1-pt.) jars, filling to 1/2 inch from top.
2. Bring vinegar and next 3 ingredients to a boil in a small saucepan over medium-high heat; boil, stirring constantly, 1 minute or until sugar dissolves. Let stand 5 minutes.
3. Pour vinegar mixture over onion mixture in jars, filling to 1/2 inch from top. Cool completely (about 20 minutes). Cover with lids, and chill 8 hours before serving. Store in refrigerator up to 2 weeks.