Photograph: Brett Stevens
For the pastry, tip the butter into the bowl of a food mixer with a pinch of salt. Beat until smooth.
Add the milk and 50ml hand-hot water; beat for 2 minutes. Tip in the flour and mix until combined.
Knead the pastry briefly on a lightly floured surface, pat into a disc; wrap in clingfilm. Chill for 30 minutes.
Trim the rhubarb and cut into 2-3cm pieces. Peel and core the apple, then chop it into bite-sized pieces. Rinse the rhubarb and mix with the apple and sugar in a wide pan.
Simmer the fruit for 8-10 minutes or until starting to soften but still in pieces. Tip the hot fruit into a sieve set over a bowl and leave to cool.
Put a pie funnel in the centre of a buttered 23-24cm pie dish, 3-4cm deep. Spoon the fruit around the funnel. Preheat the oven to 200°C, fan 180°C, gas 6.
Roll out the pastry on a floured surface and cut out a circle to cover the top of your pie, with a little extra all the way round. Brush the edge of the dish with a little of the beaten egg.
Roll the pastry around the rolling pin and then over the fruit; crimp the edges. Brush the top with egg and sprinkle with the extra caster sugar.
Bake for 30 minutes, until golden. Meanwhile, return the fruit juice to the pan; bubble to reduce by a third, pour into a jug and serve with the pie.