1. Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; pound to 1/3-inch thickness using a meat mallet or small heavy skillet. Sprinkle chicken evenly with salt and 1/4 teaspoon pepper; coat with cooking spray. Heat a large nonstick skillet over medium-high heat. Add chicken to pan; cook 5 to 6 minutes on each side or until browned.
2. While chicken cooks, cut shallots vertically into thin slices. Remove chicken from pan. Coat pan with cooking spray. Add mushrooms and shallots to pan; coat vegetables with cooking spray. Cook 1 minute, stirring constantly. Stir in sherry and marjoram. Return chicken to pan; cover and cook 3 to 4 minutes or until mushrooms are tender and chicken is done. Transfer chicken to a platter. Pour mushroom mixture over chicken; sprinkle with freshly ground pepper, if desired. Serve immediately.