Chicken with Mushroom Sauce

2 tsp cornstarch
1/2 cup milk
4 boneless skinless chicken breast halves
1 Tbsp olive oil
1/2 pound mushrooms, sliced
1/2 medium onion, sliced
1 Tbsp butter
1/4 cup sherry or chicken broth
1/2 tsp salt
1/8 tsp pepper

In a small bowl, combine cornstarch and milk until smooth; set aside.  Flatten chicken to 1/4 inch thickness.  In a large skillet, cook chicken in oil over medium heat for 5 to 6 minutes on each side or until juices run clear.  Remove and keep warm.
In the same skillet, sauté mushrooms and onion in butter until tender.  Stir in the sherry or chicken brother, salt and pepper; bring to a boil.  Stir cornstarch mixture and add to the pan.  Bring to a boil; cook and stir for 2 minutes or until thickened.  Serve with chicken.