Sift together flour, sugar and salt. Cut shortening in with pastry blender or two knives. Put egg yolk into measuring cup and add milk to make ⅔ cup. Add to shortening mixture. Mix just enough until dough shapes into a ball. Roll out ½ dough to 15" x 11" rectangle. Transfer to baking sheet. Cover with corn flakes. Mix apple sauce together with apple slices, and add mixture on top of corn flakes. Mix sugar and cinnamon. Sprinkle over apple mixture. Roll out other half of dough for top crust. Place over apples. Pinch edges together. Beat egg white until stiff and spread on top crust. Bake in hot 400°F oven for 40 minutes. Cut into 16 squares.