1Between pieces of plastic wrap or waxed paper, place each chicken breast smooth side down; gently pound with flat side of meat mallet or rolling pin until about 1/4 inch thick.
2In shallow dish, stir 1/2 cup Bisquick mix and the garlic powder. In another shallow dish, beat egg. Dip chicken in egg, then coat with Bisquick mixture.
3In 12-inch nonstick skillet, heat 2 tablespoons of the oil over medium heat. Add chicken; cook about 3 minutes or until golden brown. Turn chicken; cover and cook 4 to 6 minutes longer or until chicken is no longer pink in center. Remove to serving platter; cover to keep warm.
4In same skillet, heat remaining 1 tablespoon oil over medium heat. Add mushrooms; cook 3 to 4 minutes, stirring frequently, until browned. Add 3 tablespoons Bisquick mix and the onions; cook and stir until mixed. Stir in milk and soy sauce. Cook until mixture is thick and bubbly. Serve over chicken.