Make pastry in a food processor if you have one. Put flour, sugar and a pinch of salt in the bowl of the processor and add diced butter. Process quickly until mixture resembles coarse breadcrumbs. Pour in egg yolk and water, then process just until the mixture forms large clumps. If you don’t have a food processor, put flour, sugar and salt in a large mixing bowl and cut the butter through the flour using 2 metal knives or a pastry cutter, then rub in with the fingers until it resembles coarse crumbs. Add the mixed egg yolk and water and mix to a dough.
Turn dough onto a lightly floured surface and knead quickly into a ball. Wrap in plastic wrap and chill for 30 minutes, just until the dough is firm.
Roll out the pastry on a lightly floured surface, then transfer to a baking tray and refrigerate while preparing filling.
Preheat oven to 200°C. Peel apples, cut into quarters and remove cores. Slice thinly. Put apples in a bowl with lemon juice and toss gently until coated. Add rhubarb, then caster sugar, cornflour and cinnamon and toss gently. Pile fruit into a 20cm enamel pie plate, putting a pie bird in the centre so steam can escape. There is a lot of fruit, but it will collapse as it cooks, so pile it up and press gently to get rid of as much air as possible.
Cut a long strip of pastry from rolled-out dough. Dampen rim of pie dish and press pastry strip onto it, then dampen the top of the pastry strip with water. Cut a cross in the centre of the remaining piece of pastry and drape pastry over the fruit, letting the pie bird pop through the cut in the centre. Seal pastry edges, trim, then crimp pastry edges with the fingers. Roll out the pastry scraps and cut into leaves and other decorative pieces. Attach to the pastry with a little milk.
Brush pastry top with milk and sprinkle with sugar. Put pie plate on a baking sheet (I usually encase the bottom of the pie plate in tinfoil to catch any juices that may spill out as the pie cooks). Bake for 30 minutes until golden, then drape the pie with tinfoil and continue cooking for a further 15 minutes. Serve hot or warm with custard, cream or ice cream, if you like.
From Taste magazine, July 2009.