Put the flour on a large board or straight on to the work surface, add the icing sugar and butter, then make a dip in the centre and add the eggs. Begin to mix the eggs and butter together with your fingertips, gradually drawing the flour and sugar into the mix until it begins to clump together and you can squeeze the pastry into a ball. Knead the pastry lightly, then chill in the refrigerator for 15 minutes.
Mix together the apples, frozen mixed berries, icing sugar and cornflour for the filling.
Roll out the pastry on a lightly floured surface until it forms a rough-shaped circle about 33 cm (13 inches) in diameter. Lift it over a rolling pin on to a large greased baking sheet. Pile the fruit mix high in the centre of the pastry, then bring the edges of the pastry up and around the fruit, shaping into soft pleats and leaving the centre of the fruit mound exposed.
Brush the outside of the pie with a little milk or beaten egg and sprinkle with caster sugar. Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 20–25 minutes until the pastry is golden and the fruit tender. Serve warm or cold with custard or cream.