Heat oil in large skillet over medium-high heat until hot. Add onions; cook about 3 minutes or until soft, stirring frequently. Stir in brown sugar, vinegar and soy sauce. Reduce heat to medium; cook 6 to 8 minutes or until onions are deep golden brown and very tender, stirring occasionally.
Meanwhile, bake dinner rolls as directed on package.
To assemble sandwiches, split warm rolls. Layer caramelized onions, chicken, lettuce and tomato on bottom halves of rolls. Spread top halves with mustard; cover sandwiches with top halves of rolls.