Combine the flour and salt in a large mixing bowl. Cut the butter into small chunks. Add the butter into the flour mixture.
Use a pastry blender or a fork, cut the butter into the flour mixture. Alternatively, use two knives in a criss-crossing motion. Work fast and try not to let the butter melt. Blend the butter into the flour mixture until it has a consistency of damp sand, with a few pea-sized pieces of butter remaining.
In a separate small bowl, beat the egg with the vinegar and water together with a fork or a wire whisk. Slowly pour the egg mixture over the flour, stirring the mixture with a fork until the mixture is moist. The dough should stick together and hold the form of a small ball.
Divide the dough into 2 balls. Wrap each ball with plastic wrap and flatten it (either by hand or with a rolling pin) so it forms a flat disc that fills the corners of the plastic wrap. Chill the dough in the refrigerator for a minimum half an hour.
At this point, you can choose to finish making the pie another day. The dough can be kept in the refrigerator for up to 3 days or frozen for 3 months.
Filling
Peel, core and slice the apple thinly. Add the lemon juice into a bowl of water and toss the apples inside. This is to prevent the apples from browning.
In a separate mixing bowl, combine the sugar, cinnamon, salt, nutmeg (if using) and flour together. Drain the apples well and toss the apples into the sugar mixture. Mix well and set aside.
Assembly
Remove one dough disc from the fridge (if the dough has been chilled for a long time, soften it lightly by letting it rest at the room temperature for a couple of minutes before it's soft enough to work with).
Place a large piece of parchment paper (or baking paper) on the table top. Sprinkle the top of the parchment paper with plain flour. Unwrap the dough (remove the plastic wrap) and place the dough on top of the flour. Sprinkle the top of the dough with more flour. Dust the rolling pin with flour and roll out the dough from the centre of the disc to the edge, until the crust is the desired thickness, which is usually about 1/8-inch (3mm) thick, and at least 1 inch wider than the pie dish. Keep dusting the rolling pin while rolling to prevent the dough from sticking on to the rolling pin.
9-inch pie dish: Roll the rolled out dough onto the rolling pin and position it over the pie dish. Unroll the dough and carefully press the dough into the pie dish using your knuckles (not fingernails!) If any cracks appear during the transfer, patch it up by pushing the seams back together with lightly floured fingers.
4.5-inch pie dish: Use a 6-inch cutter (or bigger, depending on how tall your pie dish is) and cut out rounds, transfer the cut dough into the pie dish and press it in using your knuckles.
Roll out the second dough disc (also 1/8-inch thick) and set aside.
Baking
Preheat the oven to 220 degrees Celsius.
Using a fork, poke some holes in the bottom crust. Arrange the apples in the pie dish. The apples will shrink during baking, so pile the apples up high. Dot the apples with butter.
Put the top crust on the pie, pressing down the edges to seal the pie. You can crimp the edges for a decorative effect (purely optional). Make 6 - 8 slits in the top crust to allow the steam to escape during baking.
Place the pies in the centre rack. Place a baking sheet on the lower rack to catch all the apple sauce drippings.
9-inch pie: Bake for 10 minutes at 220 degrees Celsius. Lower the temperature to 175 degrees Celsius and continue to bake for 35 - 40 minutes, until the crust is golden brown and the juice is bubbling.
4.5-inch pie: Bake for 10 minutes at 220 degrees Celsius. Lower the temperature to 175 degrees Celsius and continue to bake for 25 - 30 minutes, until the crust is golden brown and the juice is bubbling.
Transfer the pies to a wire rack and cool for 1 hour. Serve warm or at room temperature.
NOTES
Use only the best-quality butter you can find for the pie crust. And cold butter is a must!
If you like vegetable shortening in the crust, use 115 grams vegetable shortening and 115 grams of unsalted butter.
If you have a food processor, by all means use it to make the dough! Combine the flour, salt and butter in the food processor; pulse several times until the mixture resembles damp sand. Don't overdo it!
Do flour the table top / counter top, dough, and the rolling pin well before rolling to prevent sticking. If the dough does stick to the table top, use a spatula / palette knife to gently ease the dough.
Pricking the bottom crust with holes and cutting slits in the top crust helps to release steam during baking, ensuring you have a crisp and flaky crust later on.
Store the baked pie in an air-tight container in a fridge. Reheat in a 150 degrees Celsius oven for 15 minutes when you want to eat it.