Melt butter in a large skillet over medium heat. Add onion and cook, stirring occasionally, 3 minutes or until tender. Add mushrooms and salt; sauté 8 minutes or until liquid evaporates and mushrooms are browned. Add vegetables and next 5 ingredients, stirring to blend.
Pour chicken mixture into a lightly greased 10" deep-dish pie plate, and cover with pastry; trim excess pastry. Bake at 375° for 30 to 35 minutes or until filling is bubbly and pastry is golden.