Prepare the gluten free puff pastry as per the recipe.You can prepare the puff pastry in advance and then keep it either in the fridge (for up to 1 week) or in the freezer (for up to 1 month).If you've kept the puff pastry in the freezer, give it enough time to thaw (either at room temperature or in the fridge overnight) before you start assembling the apple turnovers.

In a large skillet over medium-high heat, melt the butter.Tip: It’s important to use a large skillet with a large surface area instead of a pot or saucepan, because the larger surface area speeds up the moisture evaporation. This ensures that a lot of the moisture released by the apples will have evaporated by the time the apples have softened, so that the final filling won't be too wet (which could otherwise lead to leakage during baking).

Add the diced apples, sugar, lemon juice, cinnamon and salt, and stir to combine.

Cook for 5-6 minutes over medium-high heat with occasional stirring, until the apples are softened and the juices have thickened into a thick, syrupy consistency.Tip: You want the apples softened but they should still have some texture to them. It’s better to err on the firm side with the apple pieces, as they will soften even further during baking.

Allow the apple pie filling to cool to room temperature and then chill in the fridge until needed.Tip: Chilling the filling firms up the butter and therefore thickens the syrupy juices even further, which helps when it comes to assembling the turnovers.

Adjust the oven rack to the middle position, pre-heat the oven to 425ºF (220ºC) and line a large baking sheet with parchment/baking paper.

Before you start rolling out the gluten free puff pastry, make sure that it's not too cold and therefore too hard, as that can lead to it cracking as you roll it out. It should be pliable and malleable enough so that you can easily roll it out. If you press down on it with a finger, it should leave an indentation – but it should feel firm, not too soft to the touch.

On a lightly floured work surface, roll out the gluten free puff pastry into a large 14x15-inch (36x39cm) rectangle. The pastry should be about 2-3mm thin.Tip: If you’re working on a smaller work surface or if you find it easier to work with smaller pieces of pastry, you can divide the pastry in half and roll it out into two smaller 10x10½-inch (26x27cm) rectangles instead.

Divide the pastry into twelve 3½x5-inch (9x13cm) rectangles.

Transfer six of the pastry rectangles onto a large lined baking sheet.

Divide the chilled apple pie filling evenly between the six pastry rectangles (about 2-3 tablespoons of filling per each turnover). Arrange the filling in the centre of each rectangle, leaving a ½-inch (1.5cm) border free of the filling.

Brush the whisked egg lightly along the edges of the pastry.Tip: Egg washing the edges will help to seal the pastry together which in turn prevents the apple pie filling juices from leaking out during baking.

Place the other six pastry rectangles over the filling and press down along the edges to seal.

Crimp the turnovers by pressing the tines of a fork along the edges.

Brush the turnovers with egg wash, cut small slits in the top of each turnover, and sprinkle with some granulated or demerara sugar.

Place the assembled apple turnovers into the oven pre-heated to 425ºF (220ºC), close the oven doors and then immediately reduce the oven temperature to 400ºF (200ºC).Tip: The initial burst of intense heat ensures maximum flakiness and puffiness of the pastry.

Bake at 400ºF (200ºC) for 25-30 minutes or until the pastry is golden brown, puffed up and crisp.

Transfer the baked apple turnovers from the baking sheet onto a wire rack to cool. You can serve them either warm or at room temperature.

The turnovers are at their very best on the day of baking – that’s when the pastry will be perfectly crisp, so that it shatters beautifully when you bite into it.You can store the apple turnovers in a closed container for 3-4 days at room temperature but note that the pastry will soften with time as it absorbs some of the moisture from the apple pie filling. They will still be delicious, but the pastry won’t be as crisp after a few days.