First make the pastry. Mix the flour and salt in a large bowl. Grate in the butter, then rub it in with your fingertips until it looks like damp sand.
Separate the eggs, and set aside one white for later (give the other one to the dog, make a tiny meringue, or freeze it for bigger meringues another day), stir in the yolks, followed by just enough cold water to bring the mixture together into a smooth dough; four tablespoons should do it, but add it gradually.
Pinch off about a third and form both lots of pastry into a disc or rectangle, depending on the shape of your dish. Wrap well and chill for 30 minutes.
Meanwhile, peel, core and thickly slice the apples and sprinkle with lemon juice.
Heat the butter in a frying pan over a medium heat and add the apples, two tablespoons of the sugar and the alcohol, if using, then fry for about five minutes, until they’ve started to soften. Add the blackberries, turn off the heat and leave to cool completely.
Heat the oven and a baking sheet to 200C (fan 180C)/390F/390F/gas 6 and on a lightly floured surface roll, out the larger piece of pastry to about 5mm thick. Use this to line the base and side of a greased pie dish about 24cm wide, leaving an overhang around the edge, then spoon in the cooled filling.
Roll out the second piece of pastry. Beat the reserved egg white with the splash of milk, brush this all around the rim of the pastry lining the dish, then lay the lid on top, pressing together the two pieces of pastry all around the rim to seal; trim off any excess (if you like, you can roll this again and cut into shapes to stick on top).
Brush the top with more of the egg white and milk mixture, sprinkle with the remaining sugar and cut two holes in the top to allow the steam to escape. Bake on the heated baking sheet for 25 minutes, then turn down the heat to 180C (fan 160C)/350F/350F/gas 4 and bake for another 20-25 minutes, until golden. Leave to cool slightly before serving with thick cream or hot custard.