Simmer Filling: In a medium saucepan, combine chopped apples, butter, brown sugar, cinnamon, nutmeg, and lemon juice. Simmer over medium heat until apples are tender.
Thicken: Mix cornflour and cold water in a small cup. Stir into the apple mixture until thickened. Remove from heat and allow to cool completely.
Prepare Oven: Preheat your oven to 180°C (356°F) and line a baking sheet with parchment paper.
Cut Pastry: Cut 9 large rounds from the first pastry sheet. Cut the second sheet into 1.5cm wide strips for the lattice.
Assemble: Place 1 tbsp of apple filling on each round. Lay the strips across in a lattice pattern (2 parallel, 2 perpendicular).
Seal: Trim excess strips and use a fork to crimp and seal the edges of each pie.
Egg Wash: Brush the lattice and edges of the pies with the whisked egg.
Bake: Bake for 20–25 minutes until the pastry is golden brown and crisp.
Serve: Let cool slightly on a wire rack before serving.