Pie Crust: In a food processor, place the flour, salt, and sugar and process until combined. Add the butter and process for about 15 seconds.
Remove one portion of the dough from the fridge and place it on a lightly floured surface. Roll the pastry into a 12 inch (30 cm) circle. Fold the dough in half and gently transfer to a 9 inch (23 cm) pie pan. Brush off any excess flour and trim the edges of the pastry to fit the pie pan. Cover with plastic wrap and place in the refrigerator. Apple Filling: In a large bowl combine the sliced apples with the sugars, lemon juice, ground cinnamon, nutmeg, and salt. Let sit at room temperature for at least 30 minutes or up to three hours. Then, place the apples and their juices in a strainer that is placed over a large bowl (to capture the juices). Let the apples drain for about 15-30 minutes or until you have about 1/2 cup (120 ml) of juice. Spray a 4 cup (960 ml) heatproof measuring cup with a nonstick vegetable spray, and then pour in the collected juices and the 2 tablespoons (28 grams) of unsalted butter. Place in the microwave and boil the liquid, on high, about 5 to 7 minutes or until the liquid has reduced to about 1/3 cup (80 ml) and is syrupy and lightly caramelized. Remove the top pastry crust from the refrigerator and let it sit at room temperature for about 10 minutes so it has time to soften. Transfer the drained apples slices to a large bowl and mix them with the cornstarch (corn flour). Then pour the reduced syrup over the apples and toss to combine. Pour the apples and their syrup into the chilled pie crust. Moisten the edges of the pie shell with a little water and then place the top crust over the apples. Tuck any excess pastry under the bottom crust and then crimp the edges using your fingers or a fork. Using a sharp knife, make five- 2-inch (5 cm) slits from the center of the pie out towards the edge of the pie to allow the steam to escape. Cover the pie with plastic wrap and place in the refrigerator. Preheat the oven to 425 degrees F (220 degrees C). Place the oven rack at the lowest level and place a baking stone or baking sheet on the rack before preheating the oven. Place a piece of aluminum foil on the stone (or pan) to catch any apple juices. Set the pie on the stone or pan and bake for about 45 to 55 minutes or until the juices start to bubble through the slits and the apples feel tender (not mushy) when a toothpick or sharp knife is inserted through one of the slits. Make sure to cover the edges of the pie with a foil ring to prevent over browning after about 30 minutes. Remove the pie from the oven and place on a wire rack to cool for about 3-4 hours before cutting.
