1.Sift flours, custard powder and sugar into large bowl, rub in butter. Add egg yolk and enough of the water to make ingredients cling together. Press dough into ball, knead lightly on floured surface until smooth; cover, refrigerate 30 minutes.
2.Preheat oven to 220°C (200°C fan-forced). Grease 23cm pie plate.
3.To make the apple filling; peel, quarter, core and slice apples; combine in large saucepan with the water. Cover; simmer about 5 minutes or until apples are tender. Drain apples, discard liquid. Transfer apples to large bowl, gently stir in sugar, cinnamon and rind; cool.
4.Divide dough in half; roll one portion between sheets of baking paper until large enough to line base and side of pie plate. Ease pastry into pie plate. Spoon cold apple filling into pastry case; brush edge of pastry with lightly beaten egg white.
5.Roll remaining pastry until large enough to cover filling; place over filling. Press edges together to seal; trim, pinch edges decoratively. Decorate with pastry scraps, if desired. Brush pastry with a little more egg white. Sprinkle with extra sugar.
6.Bake pie 20 minutes. Reduce heat to 180°C (160°C fan-forced); bake a further 25 minutes or until pastry is browned.
Recipe can be made a day ahead; store, covered, in the refrigerator. Granny Smith apples, with their green skin and crisp, tart flesh, are ideal for this pie. They were named after Maria Anne Smith of Australia, who cultivated them from Tasmanian apple seeds.