Preheat the oven to 190°C.
Melt the butter in a large, heavy-based saucepan over a medium heat.
Combine the apple, lemon juice and sugar in a large mixing bowl and toss until evenly coated. Add to the melted butter and sauté for 3-4 minutes, tossing occasionally.
Meanwhile, roll the pastry out into a 10 inch round. Transfer to a large, round baking tray and sprinkle the ground almonds in the centre section. Top with the slices of apple, overlapping them in the centre so that you leave the outer 3-4 inches of the pastry untouched. Fold that pastry in and back towards the centre so that it covers the apples; there will be an uncovered circle in the middle with roughly a 4 inch diameter.
Brush the folded over pastry with beaten egg and bake for 25-30 minutes until the pastry is golden brown and cooked. Remove and allow to cool for a few minutes on a wire rack before transferring to a serving plate.