Saute chicken and onion in butter in a large skillet for 5 minutes. Add sliced mushrooms and following ingredients, up to and including the bay leaf. Cover and simmer 10 minutes. Remove bay leaf.
Let pie sit 10 minutes before serving.
Combine cornstarch with water in small bowl. Stir into chicken mixture and cook over medium heat, stirring constantly until mixture comes to a boil. Remove from heat and stir in soup, sour cream and cheeses.
Place prepared pastry crust into a 2 1/2 quart shallow casserole. Pour in chicken filling and top with remaining prepared pastry crust. Trim edges of pastry and seal. Pierce crust with fork in several places.
Bake at 400 for 30 to 40 minutes, or until lightly browned.