For the pastry, combine the flour, sugar, and salt in a bowl. Add the butter pieces and toss to coat. Turn out onto a clean work surface and, using a pastry cutter (or flat-bladed knife), roughly cut the butter into the flour mixture (leave some large chunks of butter as this will help the pastry to become nice and flaky as it cooks).
Create a well in the center of the flour mixture and add the vinegar water in 3 batches, working it in with your hands to form a rough dough (you may not need all of the water). Divide the dough into 2 pieces, one twice the size of the other. Shape each piece into a rough disc and wrap in plastic wrap. Chill for 3 hours.
Place the smaller piece of pastry on a lightly floured work surface and roll out to 32 cm x 26 cm (12 3/4 in x 10 1/4 in) then place on a baking tray. Roll out the larger piece of pastry until it is 6 cm (2 1/2 in) wider than the smaller piece all the way round, and place on another baking tray. Chill for 1 hour.
Meanwhile, cook the pears with the maple syrup and ginger in a saucepan over medium heat, stirring occasionally, for 25 minutes or until softened. Remove from the heat and cool completely.
To make the pies, evenly space the filling in 6 portions over the smaller piece of pastry. Lay the larger rectangle over the top and press down around each mound of filling to seal into 6 rectangular pies. Using the edge of a ruler or a blunt knife press down between each pie to make an indentation. Brush with the egg wash and scatter with the demerara sugar, then chill for 30 minutes. Preheat the oven to 200°C (400°F).
Bake for 40 minutes or until the pastry is golden and the pies are cooked through. Cool slightly then use a knife to break them into individual pies.
From The Pie Project by Phoebe Wood and Kirsten Jenkins © 2016. Photographs © 2016 by Kirsten Jenkins. Reprinted with permission from Hardie Grant Books.