Between pieces of plastic wrap or waxed paper, place each chicken breast smooth side down; gently pound with flat side of meat or rolling pin until about 1/4 inch thick.
In shallow pan, stir together flour, salt and pepper.
In 12-inch nonstick skillet, heat oil over medium-high heat.
Coat both sides of chicken with flour mixture.
Cook chicken in hot oil 6 to 8 minutes, turning once, until chicken is no longer pink in centre.
Remove chicken to serving plate; cover to keep warm.
Stir broth into skillet.
Heat to boiling over medium-high heat.
Stir in mushrooms and mustard.
Cook 2 to 3 minutes, stirring frequently, until slightly thickened.