Peel, quarter and core apples; slice thickly. Place apples, caster sugar and the water in a large saucepan; cover and bring to the boil. Reduce heat; simmer for about 10 minutes or until apples are just tender. Gently turn the apple mixture into a large colander or strainer to drain; cool to room temperature.
Preheat oven to 200C/400F. Grease a 20cm x 30cm (8-inch x 12-inch) lamington pan; line base and long sides with baking paper, extending the paper 5cm (2 inches) over sides.
Make pastry. Sift dry ingredients into a large bowl; rub in butter. Make a well in the centre. Using a knife, "cut" combined egg and milk through the flour mixture to make a soft dough.
Roll two-thirds on a floured surface until able to cover the base and sides of the pan with a 1 cm (1/2-inch) overhang. Lift into pan; spread with apple mixture. Brush edges with milk. Roll remaining pastry until able to cover pie. Place over filling; press edges together to seal. Trim away excess pastry. Brush top with milk; sprinkle with white sugar. Slash about six holes in the pastry.
Bake pie for 45 minutes. Stand in pan for 10 minutes; transfer, top-side up, to a wire rack to cool.
Meanwhile, make passionfruit icing. Sift icing sugar into medium heatproof bowl; stir in enough passionfruit pulp to make a thick paste. Place bowl over a saucepan of simmering water; stir icing until spreadable.
Spread icing over pastry.