Preheat oven to 200°C/400°F/Gas Mark 6. Sift the flour and Sugar Free into a mixing bowl. Add diced butter to flour and rub in with fingertips till the mixture resembles coarse breadcrumbs.
Sprinkle five tablespoons of chilled water and mix to make a dough without kneading much. Divide the dough into two equal portions and shape each into a ball.
Roll out one portion on a lightly floured surface to a disc of about twelve inches in diameter. Use this to line a nine-inch pie dish, easing the dough in carefully so that it does not get stretched.
Trim off the excess dough. Roll out the remaining dough to a rectangle of one-eighth inch thickness. With a sharp knife, cut out half inch strips.
Cook apples with half a cup of water till all the moisture is absorbed. Set aside to cool. Add cinnamon powder and mix. Take the lined pie dish, spread the apple mixture over it and sprinkle Sugar Free over the apples.
Cover it with dough strips in a crisscross pattern. Brush the strips with egg yolk and bake in the preheated oven for forty-five minutes.
Let the pie cool in the dish. Cut into wedges and serve.