BRING water to boil in large saucepan. Add onions; cook 5 min. Drain. Place onions in medium bowl. Add sugar, vinegar and pepper; mix well. Cover. Refrigerate until ready to use, stirring freqently.
MEANWHILE, combine chicken and barbecue sauce in saucepan; cook on medium heat 5 min. or until heated through, stirring frequently.
SPOON about 1/2 cup of the chicken mixture onto bottom half of each roll; top with 1 Singles and 1/4 cup of the onions. Cover with tops of rolls.