Combine flour, salt, and butter like pie crust. (Grandma T. uses ½ c. butter and ½ c. margarine instead of 1 c. butter) Combine egg and milk; add to flour mixture. Make 2 balls from the crust mixture; roll thin to fit a 15x10-inch jelly roll pan. Put 1 crust on bottom; fill with apple mixture (apple slices, sugar, cinnamon, and flour). Cover with second crust. Seal edges. Bake in 350° oven 35-40 minutes. Frost when cool with powdered sugar icing.