Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; pound to1/2 inch thickness using a meat mallet or small heavy skillet.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Sprinkle salt and pepper over chicken. Add chicken to pan; cook 5 minutes on each side or until done. Remove chicken and drippings from pan; keep warm.
Heat pan over medium-high heat; recoat pan with cooking spray. Add mushrooms to pan. Coat mushrooms with cooking spray; cook 2 minutes or untill tender, stirring frequently. Add wine and 3 tablespoons onions to pan; cook 30 seconds over high heat. Reduce heat; add butter, stirring until butter melts.
Add chicken and drippings to pan, stirring gently. Spoon mushroom sauce over chicken; sprinkle with remaining onions.