Roll out the pastry on a lightly floured surface to a circle, roughly 35cm in diameter. Use a rolling pin to lift the pastry onto a large baking tray lined with baking paper and chill in the fridge for 20 minutes.
Preheat the oven to 200°C, fan 180°C, gas 6. Layer the sliced apples and 2 tablespoons of the caster sugar in the centre of the pasty circle, leaving a 6cm border all around. Sprinkle over half the cinnamon. Gently fold in the pastry border to partially enclose the apples. Brush the pastry border with the beaten egg. Sprinkle the rest of the sugar and cinnamon over the pastry border.
Bake in the oven for 40-45 minutes until the pastry is crisp and golden. Heat the Ambrosia custard following the pack instructions and serve with the pie.