Pre-heat the oven to 180Cfan.
In a food processor, pulse the dry ingredients, then add the butter diced into small pieces. Pulse briefly until it resembles coarse breadcrumbs. Now add the iced water to form a dough, pulsing gently to mix. (Don't over mix). Tip it out onto the work surface and bring it together to form a smooth dough. Wrap and pop it in the fridge to rest for 30 minutes.
In the meantime peel and core the apples and slice finely. Wash the blackberries and place in a bowl, add the sugar, cornflour, lemon zest and juice. Stir and set to one side.
Once the pastry has chilled have a baking tray ready lined with parchment paper. If you’re not confident with pastry you can start rolling out the pastry on the parchment paper. Sprinkle a little flour on the surface. You're aiming for a disc shape however this is a rustic tart so it doesn’t have to be precise.
Once you have a disc approximately 32cm in diameter start arranging the apple slices and blackberries in the centre, leaving approximately a 6cm gap from the edge, making sure you have an even layer.
Now fold the edges over. Brush with beaten egg and sprinkle with granulated sugar.
Bake on the centre shelf for 35-40 minutes, or until golden. Serve warm with cram or ice-cream.