First, make your pastry dough, wrap it in plastic wrap and rest it in the refrigerator for at least half an hour. Then preheat the oven to 350 degrees F.
Put the butter and sugar into a saucepan and, when the butter has melted, add the apples, ginger and a tablespoon of the ginger syrup. Slowly cook for 15 minutes with a lid on, then add the blackberries, stir and cook for 5 more minutes with the lid off.
Meanwhile, remove your pastry from the refrigerator. Dust your work surface with flour, cut the pastry in half and, using a floured rolling pin, roll 1 of the pieces out until it's just under 1/2-inch thick or looks as if it will cover a shallow 10-inch pie dish. (Rolling the dough between 2 layers of waxed paper will also stop it sticking to your rolling pin.) Butter the shallow 10-inch pie dish and line with the pastry, trimming off any excess around the edges using a sharp knife.
Tip the cooled apples and blackberries into a sieve, reserving all the juices, then put the fruit into the lined pie dish so you have a mound in the middle. Spoon over half the reserved juices, (if desired use all the juice). Brush the edge of the pastry with beaten egg. Roll out the second piece of pastry, just as you did the first, and lay it over the top of the pie. Trim the edges as before and crimp them together with your fingers. Brush the top of the pie with the rest of the beaten egg, sprinkle generously with sugar and the cinnamon, and make a couple of slashes in the top of the pastry.
Place the pie on a baking tray and then put it directly on the bottom of the preheated oven for 55 to 60 minutes, until golden brown and crisp. To serve, slice the pie into portions and serve with a generous dollop of custard.
Sift the flour from a height onto a clean work surface and sift the icing sugar over the top.
Using your hands, work the cubes of butter into the flour and sugar by rubbing your thumbs against your fingers until you end up with a fine, crumbly mixture. This is the point where you can spike the mixture with interesting flavors, so mix in your lemon zest.
Add the eggs and milk to the mixture and gently work it together until you have a ball of dough. Flour it lightly. Don't work the pastry too much at this stage or it will become elastic and chewy, not crumbly and short. Flour your work surface and place the dough on top. Pat it into a flat round, flour it lightly, wrap it in plastic wrap and put it into the refrigerator to rest for at least half an hour.
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Yield: about 2 pounds