To prepare dressing, combine first 6 ingredients in a small bowl, stirring with a whisk. Cover and chill.
To prepare sandwich, combine 1 tablespoon mint and next 4 ingredients (through garlic) in a large zip-top plastic bag. Add chicken breast tenders; seal and marinate in refrigerator for 15 minutes, turning occasionally.
Remove chicken from bag; discard marinade. Place chicken and onion on grill rack coated with cooking spray. Grill 4 minutes on each side or until chicken is done and onion is tender. Remove from grill; sprinkle chicken with salt and pepper.
Grill roll halves, cut side down, until lightly toasted, about 1 minute.
Spread about 1 tablespoon dressing over the bottom halves of the rolls. Top each serving with 3 ounces chicken breast tenders, 1 onion slice, 2 tomato slices, and 1 lettuce leaf; cover with top halves of rolls.