For crust: Mix together the shortening, sugar, flour and salt with a fork or pasty blender as for pie crust. Mix yolk and milk together and add to flour-shortening mixture. Blend. Divide dough into two parts and roll out a little thicker than pie dough (about 1/4 inch) into a roughtly 13-by-8-inch rectangle and place in an ungreased jelly roll pan about 10-by-15-inches. The dough is sticky, so you will need a generously floured board.
For filling: Arrange apple slices on the dough, then sprinkle with sugar, flour and cinnamon that has been mixed together. Cover with the second rectangle of dough and pinch the edges together all the way around. Bake at 400 degrees F. for 15 minutes, then reduce heat to 325 degrees F. and continue baking for about 25 minutes. Remove from oven and frost immediately with a powdered sugar frosting (see below). The frosting will melt into the pastry, creating a glaze. Cut into rectangles, with diagonal cross-cuts for a pretty look..
Powdered sugar frosting: Mix together about one cup of powdered sugar, 1 tablespoon of butter, 1/4 teaspoon almond extract and enough milk to make a nice spreading consistency.