First prepare the filling. Put the cherries, sugar, lemon juice and water in a saucepan and cook over medium heat for about 10 minutes until the cherries are tender. Add the cornflour paste to the cherries and cook until the sauce thickens. Allow the cherry filling to cool to room temperature.
Preheat oven to 190°C.
Roll out the puff pastry onto Spoon the cherry mixture on top of the puff pastry sheet, leaving an inch boarder. Fold over the edges to create a rim.
Brush the rim with the beaten egg.
Bake for 18 to 20 minutes until pastry is golden brown.
Serve warm or cooled with some icing sugar dusted over it.