In a small bowl, combine cornstarch and milk until smooth; set aside. Flatten chicken to 1/4 in. thickness. In a large nonstick skillet, cook chicken in oil over medium heat for 5-6 minutes on each side or until juices run clear. Remove and keep warm.

In the same skillet, saute mushrooms and onion in butter until tender. Stir in the sherry, salt and pepper; bring to a boil. Stir cornstarch mixture, add to the pan. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with chicken.