In a mixing bowl, combine 3 cups flour, salt, and shortening. Cut in shortening until mixture resembles course crumbs. Combine egg yolk (reserve egg white) and milk; add to flour mixture, stir just until dough clings together. Divide dough in half. On a slightly floured surface, roll half of dough into a 15x10” rectangle; transfer to a greased 15x10” baking pan. Set aside. In a bowl, toss together the apples, sugar, butter, cinnamon, and the remaining 2 Tbsps. of flour. Spoon over pastry. Roll out remaining dough to another 15x10” rectangle. Place over filling. Brush with egg white. Bake at 375 degrees for 35-40 minutes or until golden brown. Cool on wire rack. Combine powdered sugar and water. Drizzle over warm pastry. Serve warm or cold.