Prepare filling: Preheat oven to 350. Melt 1/3 c butter in large saucepan over medium heat; add all-purpose flour, and cook, whisking constantly. 1 minutes. Gradually add chicken broth and milk, and cook, whisking constantly, 6 to 7 minutes or until thickened and bubbly. REmove from heat, and stir in Creole seasoning.
Melt 2 tbsp butter in a large dutch oven over medium-high heat; add onion and mushrooms, and saute 10 minutes or until tender. Stir in chicken, next 4 ingredients, and sauce.
Prepare crust. Place 1 piecrust in a lighlty greased 10 in cast iron skillet (or 4qt casserole). Spoon chicken mixture over piecrust, and top with remaining piecrust.
Spray pie crust with butter spray. Sprinkle lightly with garlic powder. Cut 4 to 5 slits in top of pie for steam to escape
Bake at 350 for 1 hour to 1 hour and 5 minutes or until golden brown and bubbly.