1. Preheat grill to medium-high heat.
2. Combine sliced onion, 1 tablespoon vinegar, and 1 1/2 teaspoons honey in a small bowl; toss well. Cover and refrigerate onion mixture at least 30 minutes.
3. Combine remaining 2 tablespoons vinegar, remaining 1 tablespoon honey, 2 tablespoons water, mustard, and 1/4 teaspoon pepper in a small saucepan over medium-low heat. Bring to a simmer (do not boil). Simmer 5 minutes; stir in butter. Remove from heat; keep warm.
4. Sprinkle chicken with remaining 1/2 teaspoon pepper and salt. Place on a grill rack coated with cooking spray; grill 4 minutes on each side or until done. Remove chicken from grill; cool slightly. Shred chicken into bite-sized pieces. Add chicken to saucepan with mustard mixture, and toss gently to coat.
5. Cut rolls in half crosswise. Place rolls, cut sides down, on grill; grill 1 minute or until toasted. Spoon about 1/3 cup chicken mixture over bottom half of each roll. Top each roll with about 1 teaspoon drained onion slices and top half of roll. Serve immediately.