1. Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; pound to 1/2-inch thickness using a meat mallet or small heavy skillet.
2. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Sprinkle chicken evenly with salt and pepper. Add chicken to pan. Cook 5 to 6 minutes on each side or until done. Remove chicken and drippings from pan; set aside, and keep warm.
3. Heat pan over medium-high heat; coat pan with cooking spray. Add mushrooms. Coat mushrooms with cooking spray; cook 2 minutes or until tender, stirring frequently. Add wine and 3 tablespoons onions. Cook 30 seconds over high heat. Reduce heat; add butter, stirring until butter melts.
4. Add chicken and drippings to pan, stirring gently. Place chicken on platter. Spoon mushroom sauce over chicken; sprinkle with remaining onions.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.