Melt the butter and sugar into a saucepan gently and then add the braeburn and bramley apples. Slowly cook for 10 minutes with the lid off, then add the blackberries, stir and cook for 2 more minutes on low. Any longer and the blackberries fall apart. Then take off the heat and allow to cool. If the apples are still hot, when you put the apples on the pastry, the pastry will start to melt before you put it in the oven and the pie will have a soggy bottom.
Make your pastry dough, wrap it in cling film and rest it in the fridge for at least an hour. I left it in the fridge for 2 hour. I think any longer than that you might need to take it out for a few minutes to warm up a bit before rolling.
Remove the pastry from the fridge. Dust your work surface with flour, cut the pastry in half and, using a floured rolling pin, roll one of the pieces out until it’s just under 1cm thick. Lightly grease a pie dish (mine was a shallow onand line with the pastry, trimming off any excess round the edges using a sharp knife.
Then preheat the oven to 180°C/350°F/gas If you have a pie funnel put that in the middle of the pie dish. Tip the cooled apples and blackberries around it into the pastry lined pie dish. (There shouldn't be too much juice but if the cooled apples and blackberries have a lot of liquid tip them into a sieve, with a bowl underneath to catch all the juices, then put the fruit into the lined pie dish. Spoon over half the saved juices.)
Brush the edge of the pastry with beaten egg. Roll out the second piece of pastry for the lid and lay it over the top of the pie making a hole in the middle and gently put the pastry over the pit funnel. Trim and press the edges together. Brush the top of the pie with the rest of the beaten egg, sprinkle generously with sugar, cinnamon and nutmeg.
Place the pie in the oven for 50 to 60 minutes, until golden brown and crisp. Serve still hot with lots of custard.