Line a baking sheet with parchment paper, set aside. In a large pan, combine the cherries, lemon juice, sugar & vanilla. Cook over a medium heat until the cherries soften & release their juices, about 5 minutes.
In a small bowl, whisk the cornflour & water together, & quickly stir it into the cherry juices to thicken. Stir constantly & cook for one more minute before setting aside to cool.
Roll the pastry out on a lightly floured surface to 10×15-inches. Cut into six 5-inch squares & place a generous spoonful of the cooled cherry mixture in the centre of each turnover. Yossy warns not to overfill the turnovers as this makes them super hard to seal, & she’s totally right, be careful! Brush the edges of the dough with the beaten egg & fold each square diagonally into a triangle. Press the edges together with a fork to seal them & transfer to the lined baking sheet & freeze for about 15 minutes.
Preheat the oven to 220C & place rack in the centre of the oven.
When ready to bake, brush the turnovers with the remaining beaten egg & sprinkle with the demerara sugar & a few grain of sea salt flakes. Use the tip of a sharp knife to cut a small slit into the top of each turnover & bake for about 20-25 minutes, or until a deep golden brown in colour. Cool slightly before serving!