1. Place onions in a large bowl, and sprinkle with 3 tablespoons sugar. Set aside.
2. Combine vinegar, salt, and remaining sugar in a saucepan over medium heat. Cook, stirring frequently, until sugar dissolves. Add pepper; cook 2 minutes. Remove pepper.
3. Bring mixture to a boil; stir in reserved onions. Remove pan from heat, and let stand 20 minutes. Cover and chill until ready to serve.