Preheat the oven to 180°C/Fan 160°C/Gas 4. To make the pastry, cream the butter and sugar together by hand or in a food processor. Add the eggs one by one and beat for several minutes. Reduce the speed and mix in the flour slowly. Turn out onto a piece of floured baking parchment, flatten into a round, then wrap and chill. This pastry needs to be chilled for at least 2 hours otherwise it is difficult to handle – better still, make it the day before.
Roll out the pastry to about 3mm thick, then use about two-thirds of it to line a 18 x 30 x 2.5cm square tin or a 22.5cm round tin. Fill the pie to the top with the apples and blackberries and sprinkle with the sugar. Cover with a lid of pastry, press the edges together to seal. Decorate with pastry leaves, brush with the beaten egg mixture and bake for 45 minutes–1 hour until the apples are tender.
When cooked, sprinkle lightly with caster sugar, cut into pieces and serve with softly whipped cream and sugar.