Preheat oven to 425 degrees F.
Heat margarine over low heat until melted. Blend in flour, onion, paprika salt and pepper. Cook over low heat, stirring constantly, until mixture is smooth and bubbly; remove from heat.
Stir in broth and milk. Heat to boiling, stirring constantly. Boil and stir 1 minute.
Stir in chicken and frozen vegetables; reserve.
Prepare Celery Seed pastry. Roll 2/3 of pastry into 13-inch square on lightly floured cloth-covered board. Ease pastry into square pan, 9x9x2 inches; pour chicken filling into pastry-lined pan. Roll remaining dough into 11-inch square; place over filling. Roll edges under; flute. Cut slits in center to allow steam to escape.
Bake, uncovered, in 425 degrees F oven until crust is brown, 30 to 35 minutes.
Cut shortening into flour, celery seed and salt until particles are size of small peas. Sprinkle in water, 1 Tablespoon at a time, tossing with fork until all flour is moistened and pastry almost cleans side of bowl (1 to 2 teaspoons water can be added if necessary). Gather pastry into ball.
*If using self-rising flour, omit salt.