COOK mushrooms and onions in large nonstick skillet on medium-high heat 10 min., stirring occasionally.

ADD dressing, garlic and thyme; cook 5 to 7 min. or until mushrooms are tender, stirring frequently. Transfer to bowl; cover to keep warm. Add chicken to skillet; cook 5 min. on each side or until done (170ºF).

TOP chicken with mushrooms and cheese; cover. Cook 2 to 4 min. or until cheese is melted; sprinkle with chives.