Place each chicken breast between two pieces of plastic wrap.
Working from the center to the edges of each piece, lightly pound it with the flat side of a meat mallet to a 1/8-inch thickness.
Remove plastic wrap. Combine flour, salt and pepper in a flat dish. Then lightly press chicken into this mixture to coat both sides.
Sauté mushrooms and scallions until tender in a skillet treated with nonstick cooking spray.
Transfer mushroom and scallions to a bowl, then brown the chicken in skillet over medium-high heat.
Return the mushroom mixture to pan and carefully add wine then the broth.
Cook uncovered for 2-3 minutes or until sauce thickens slightly, then reduce heat and simmer to desired consistency.
Serve chicken topped with mushrooms and sauce.