HEAT 1 Tbsp. oil in skillet on medium heat. Add chicken; cook 6 to 7 min. on each side or until done (165ºF). Remove to platter; cover to keep warm.
ADD remaining oil and onions to skillet; cook and stir 3 to 5 min. or until onions are tender. Add mushrooms; cook and stir 2 min.
MIX remaining ingredients until blended. Pour into skillet; cook on low heat 3 to 5 min. or until heated through. Pour sauce over chicken.