Place chicken in a greased 2 quart baking dish cover with water. Cover and bake @ 350 for 30 minute or until juices run clear. Meanwhile, in a skillet, saute’ mushrooms and onions in butter until tender. Combine cornstarch and milk until smooth. Stir into skillet. Add seasonings. Bring to boil. Cook and stir for 2 min or until thickened.
Drain chicken. Cut pastry sheets in half widthwise. On one side of each half place chicken breast, ¼ cup mushroom mixture, 2 ham slices, and one cheese slice. Fold pastry over filling and seal edges.
Place each on greased baking sheet. Brush tops with milk if desired. Bake @ 400 for 15-20 minute or until puffed and golden.