Preheat your oven to 400°F and line your baking sheets with parchment paper.

In a saucepan over medium heat, melt the butter, then add your diced apples, brown sugar, cinnamon, salt, and lemon juice. Stir everything together.

Keep cooking the apple mixture over medium heat, stirring occasionally, until the apples soften and the liquid starts reducing down, about 8 to 10 minutes.

In a small bowl, mix your cornflour with the water until it forms a smooth, lump-free slurry.

Stir the cornflour slurry into your softened apple mixture and continue cooking for about 2 minutes until the filling thickens to a glossy, jam-like consistency.

Transfer your apple filling into a bowl and let it cool completely before using.

On a lightly floured surface, roll your puff pastry out to about 1/8 inch thickness.

Cut your rolled pastry into 8 rectangles, each roughly 4.5 by 6 inches.

Place 1 to 2 teaspoons of your cooled apple filling in the center of each pastry rectangle.

Brush the edges of each rectangle with egg wash or milk, then fold the pastry over to enclose the filling. Press the edges together firmly and crimp with a fork.

Using a small knife, cut a few small slits across the top of each sealed pie.

Brush the tops of each pie with egg wash, then sprinkle cinnamon sugar evenly across the surface.

Bake your pies for 25 to 30 minutes, watching for a deep golden color across the tops.

Let the pies cool for a few minutes before serving warm.