Make the apple filling. Put apples, juice, sugar, cinnamon and ginger into a saucepan and bring to a simmer over medium heat. Cook, stirring occasionally, until the apples are tender and the liquid has reduced a little, around 10-15 minutes.
Mix the cornstarch with a couple of tablespoons of water to dissolve, then pour this into the pan, stirring quickly and well so there are no lumps. Simmer, stirring, for a couple more minutes to cook the starch and thicken the mixture.
Remove from heat, then remove and roughly mash half of the apples with a fork. Add the unmashed apples and liquid, stir well, then place in the fridge to chill.
Make the turnovers. Line a cookie sheet or shallow baking tray with parchment (baking) paper and set aside.
On a lightly floured surface, roll the pastry to about 1/8 inch (3mm) thickness. Cut into eight 5 inch (12½ cm) circles using a large round cutter or a small plate as a size guide.
Working quickly so the pastry doesn’t get warm, divide the chilled apple filling between the pastry discs, placing it over one half of each circle and leaving a ¼ inch (1½ cm) border. Brush the border lightly with egg, then fold the pastry over to make semi-circle shapes. I find it easiest to do this by carefully lifting up the pastry with the filling on it, then bringing the sides upwards and pinching closed all the way around. As you make each turnover, move it to the lined tray and crimp the edges with the tines of a fork.
Once all the turnovers are made, brush the tops with a little extra egg and sprinkle with sugar (optional). Put the tray in the fridge to chill for at least an hour.
Bake the turnovers. Preheat oven to 350°F/180°C, combination steam setting. If your oven has variable steam, use 50% (if not, don’t worry! Just set to combination or convection steam and the oven will figure out the steam level for you).
When the oven is hot, put the turnovers in and bake until they’re golden brown and puffed all over, about 20 minutes. Allow to cool on the tray for 10 minutes, then move to a cooling rack. Serve the turnovers warm or at room temperature. They’re brilliant as they are, but if you want to turn them into a special dessert, serve with vanilla ice cream, whipped cream or custard.