Melt margarine or butter in a large saucepan over low heat. Add flour, onion, salt, and black pepper and cook, stirring constantly, until smooth and bubbly. Remove from heat and stir in chicken broth and milk. Bring to a boil while stirring constantly, then cook while continuing to stir for about one minute or until the mixture thickens. Stir in chicken, vegetables, and potatoes.
Line a pie pan with the bottom crust and pour in the filling. Cover with the top crust, seal the edges, and cut away any excess dough. Cut small slits in the center to vent.
Cook for 30-35 minutes or until the crust is golden brown and the filling is bubbly. Cool for 10 minutes before serving.