Preheat oven to 450°. In a large saucepan, heat oil over medium-high heat. Add onion; cook and stir until tender. Stir in flour and poultry seasoning until blended; gradually whisk in broth and milk. Bring to a boil, stirring constantly; cook and stir 2-3 minutes or until thickened. Stir in chicken and vegetables.Transfer to a greased 9-in. deep-dish pie plate; place pastry sheet over filling. Trim, seal and flute edges. Cut slits in pastry. Bake 15-20 minutes or until crust is golden brown and filling is bubbly. Yield: 6 servings.