1. Cook first 4 ingredients in a Dutch oven over low heat, stirring occasionally, until sugar dissolves. Increase heat to medium, and cook 10 minutes. Pour mixture through a wire-mesh strainer into a bowl, discarding mint sprigs. Return vinegar mixture to Dutch oven, and bring to a boil. Add onion rings; bring to a boil, and boil 1 minute. Remove onion mixture from heat, and stir in diced pimiento. Cool 30 minutes.
2. Stir 2 Tbsp. chopped mint into onion mixture; cover and chill at least 2 hours.