1. Cook first 6 ingredients in a large saucepan over high heat, stirring occasionally, about 3 minutes or until mixture is hot and sugar dissolves. (Do not boil.)
2. Place cucumbers and onions in a 4-qt. airtight plastic container. Pour hot vinegar mixture over cucumbers and onions. Cool 30 minutes. Serve immediately, or refrigerate in airtight container up to 2 weeks.