Cook and drain vegetables as directed on bag.
Heat oven to 375°F. In 2-quart saucepan, mix flour, salt, poultry seasoning, pepper and milk with wire whisk until blended. Stir in onion. Cook over medium heat 4 to 6 minutes, stirring constantly, until thickened.
Stir in soup and sour cream. Add chicken and cooked vegetables; mix well. Cook, stirring frequently, until thoroughly heated. Pour into ungreased 2-quart round casserole. Unroll pie crust; place over hot filling. Seal edge and flute as desired. Cut slits in several places in crust.
Bake 35 to 40 minutes or until crust is golden brown and mixture is bubbly. Let stand 10 minutes before serving.