1. Soak onion slices in ice water to cover in a small bowl 10 minutes; drain. Place onion slices and bell pepper strips in 1 (1-qt.) canning jar.
2. Bring vinegar, next 3 ingredients, and 1 cup water to a boil in a small nonaluminum saucepan over medium-high heat, stirring occasionally, until sugar is dissolved.
3. Pour hot vinegar mixture over vegetables in jar. Let stand, uncovered, 1 hour. Cover and chill 24 hours. Store in an airtight container in refrigerator up to 1 week.