Roll out the puff pastry

Preheat the oven to 425°F(220°C). Line a baking sheet with parchment (baking) paper. Place the puff pastry sheet on a lightly floured work surface and roll out into a 10-by-15-inch (25-by-38-cm) rectangle. Cut in half lengthwise and then cut each half crosswise into 3 squares to make a total of 6 squares. Place the squares on the prepared baking sheet, spacing them evenly.

Prepare the cherry filling

Place the cherries in a bowl and sprinkle with the brandy, if using. Add the granulated sugar and flour and toss.

Fill and bake the turnovers

In a small bowl, whisk together the egg and milk. Brush a ½-inch (12-mm) border of the egg mixture around 2 adjacent sides of each square. Spoon about 3 tablespoons of the cherry filling almost in the middle of each square. Fold the pastry over the filling to make a triangle and press the free edges with a fork to seal. Brush the tops with the remaining egg mixture and sprinkle with the coarse sugar. Bake until the turnovers are puffed and golden brown, 15–18 minutes. Let cool slightly on the baking sheet on a wire rack. Serve warm.