PREHEAT oven to 400°F. Cook and stir onion in margarine in large saucepan on medium-high heat until crisp-tender. Blend in flour; cook and stir 1 minute. Add beef broth and steak sauce; cook and stir until mixture thickens and begins to boil. Stir in steak and peas and carrots. Spoon into 2-quart casserole dish.
ROLL out pastry crust to size about 1-1/2 inches larger than size of casserole dish. Place pastry over casserole; turn under edge of pastry and press to edge of casserole dish to seal. Flute edge, if desired. Cut several slits in center of crust to vent.
BAKE 25 to 30 minutes or until golden brown. Serve warm.