Melt butter in a large frying pan; cook apples, stirring, 10 minutes or until apples soften. Add brown sugar; stir until caramelised. Remove from heat; cool.

Meanwhile, make pastry.

Bring rhubarb, caster sugar and half the water to the boil in a large saucepan. Reduce heat; simmer, covered, 5 minutes or until rhubarb is tender.

Blend cornflour with the remaining water; stir into rhubarb mixture. Stir over heat until mixture boils and thickens. Remove from heat; fold rhubarb mixture into apple mixture. Cool 20 minutes.

Grease a deep 24cm (9-inch) round springform tin. Roll three-quarters of the pastry between sheets of baking paper until large enough to line tin. Ease pastry into tin, press into base and side; trim edge. Refrigerate 30 minutes, with remaining pastry
and any scraps.

Preheat oven to 200°C.

Spoon cold apple and rhubarb mixture into pastry case; brush edge with egg. Roll remaining pastry out until 3mm (⅛-inch) thick, cut into 1cm (½-inch) strips. Twist strips, place over filling in a lattice pattern; trim ends. Brush top with remaining egg; sprinkle with demerara sugar.

Bake pie 20 minutes. Reduce oven to 180°C/350°F; bake further 25 minutes. Stand 15 minutes.

Process flours, sugar and butter until crumbly. With motor operating, add egg and enough of the water to make ingredients come together. Knead dough on a floured surface until smooth. Wrap in plastic wrap; refrigerate 30 minutes.