Preheat oven to 400 degrees F. In a large saucepan melt butter over medium heat. Add leek, mushrooms, celery, and sweet pepper; cook for 5 to 8 minutes or until vegetables are tender. Stir in flour, poultry seasoning, salt, and black pepper. Add chicken broth and half-and-half all at once. Cook and stir until thickened and bubbly. Stir in chicken and peas. Pour into an ungreased 3-quart rectangular baking dish.
Place Pastry Topper over chicken mixture in dish. Brush pastry with egg.
Bake, uncovered, for 35 to 40 minutes or until pastry is golden. Let stand for 20 minutes before serving.
In a large bowl stir together flour and salt. Using a pastry blender, cut in shortening until pieces are pea-size. Sprinkle 1 tablespoon cold water over part of the mixture; gently toss with a fork. Push moistened dough to side of bowl. Repeat, using 1 tablespoon cold water at a time, until all of the flour mixture is moistened (7 to 9 tablespoons cold water total). Form dough 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, or if desired, use a small cookie cutter to cut shapes from pastry.