Prepare pastry and set aside.
In a saucepan, cook onion in butter or margarine until tender.
Stir in flour, salt, sage or thyme, and pepper. Add chicken broth and milk all at once.
Cook and stir until thickened and bubbly.
Stir in chicken and vegetables; heat until bubbly. Pour chicken mixture into a baking dish.
If using a rectangular dish, roll pastry into a rectangular shape large enough to overlap the edges of your dish.
Place pastry over the baking dish and flute the edges. Cut slits in the top to allow steam to escape.
Bake at 450 for 12 to 15 minutes, or until pastry is golden brown.