Place each chicken breast half between two pieces of plastic wrap. Pound with the flat side of a meat mallet until an even thickness (1/8 to 1/4 inch). Season lightly with salt and pepper. Coat both sides with flour.
In a 12-inch skillet, cook half of the chicken in 1 tablespoon of the butter or margarine for 4 minutes or until the chicken is no longer pink, turning once. Remove from skillet. Repeat with remaining chicken and butter.
Add mushrooms and leek to the skillet. Stir in broth and sherry. Bring mixture to boiling and cook for 8 to 10 minutes or until the liquid is reduced to a glaze consistency.
Return chicken to pan; heat through. Serve chicken topped with mushroom mixture. Makes 4 servings.