In a large saucepan cook leeks, mushrooms, celery, and sweet pepper in hot butter over medium heat for 4 to 5 minutes or until tender. Stir in the flour, poultry seasoning, salt, and black pepper. Add broth and milk all at once. Cook and stir until thickened and bubbly. Stir in the cooked turkey and frozen peas. Pour into a 2-quart rectangular baking dish.
Place pastry over the hot chicken mixture in dish. Turn edges of pastry under; flute to the edges of dish. Brush with some of the beaten egg. If desired, place reserved pastry shapes on top of pastry. Brush again with egg.
Bake, uncovered, in a 400 degree F oven for 30 to 35 minutes or until crust is golden brown. Let stand about 20 minutes before serving. Makes 6 servings.
In a medium bowl stir together flour and salt. Using a pastry blender, cut in shortening until pieces are the size of small peas. Sprinkle 1 tablespoon cold water over part of the mixture; gently toss with a fork. Push moistened dough to side of bowl. Sprinkle 3 to 4 tablespoons more cold water over remaining flour mixture, 1 tablespoon at a time, tossing with a fork until all dough is moistened. Form into a ball. On a lightly floured surface, roll dough into a 13x9-inch rectangle. Using a sharp knife, cut slits in pastry to allow steam to escape. If desired, use a small cookie cutter to cut shapes from pastry.