In a large skillet, brown chicken over medium heat in oil. Cover and cook for about 20 minutes or until a thermometer reads 170°. Remove chicken; keep warm. In the same skillet, saute mushrooms and onions until tender. Stir in the wine, butter, salt and pepper., In a small bowl, combine cornstarch and water until smooth; add to skillet. Bring to a boil; cook and stir for 2 minutes or until thickened. Return chicken to skillet; heat through.