In a large bowl, combine all the filling ingredients, except the cornstarch. Toss to mix well. Let sit at room temperature for 1 hour.
Drain the filling over a 2 to 2 1/2 quart saucepan, pressing down on the fruit gently and catching the liquid- you should have at least 1/2 cup of liquid. Return the apples to their bowl. Set the pan over high heat and reduce the liquid to about 1/3 cup which will take about 5 minutes. You can swirl the pan occasionally, but avoid stirring. Remove the pan from the heat and set aside to cool for 5 minutes.
Add the cornstarch to the apples and toss to coat. Pour the apple syrup over the fruit and stir gently to blend.
Position a rack to the center of the oven and preheat it to 425 F. Line a baking sheet with aluminum foil. Spray a 9 inch glass pie plate with nonstick cooking spray and dust with flour, knocking out the excess. Place the pie plate on the lined baking sheet.
On a lightly floured surface, roll out 1 dough disk into a 12 inch circle. Place the dough in the prepared pie dish. In a small bowl, combine the flour and 2 teaspoons of the sugar. Scatter evenly across the bottom of the dough. Scrape the filling into the crust.
Roll out the second dough disk to a 12 inch circle. Drape the dough over the filling. With a thin, sharp knife, slice a few vents around the center of the pie. Press to seal the top and bottom crust edges together. Fold the overhand under; crimp decoratively. Freeze the pie until the top crust is firm, about 15 minutes.
In a small cup, whisk together the egg, water and salt. Brush lightly over the top crust, and sprinkle with remaining 4 teaspoons of sugar.
Bake the pie at 425 F for 15 minutes. Rotate the pie 180 degrees. Lower the oven temperature to 375 F. Bake for another 50 to 60 minutes, until the filling is bubbling (bubbling is key to indicate that the starch has begun to thicken the filling!) and the crust is deeply browned. If the pie's edges look as if they may burn at any point during the end of the baking time, simply roll the foil from the pan up over the pie's edges.
Allow the pie to cool completely at room temperature on a wire rack before slicing and serving, at least 3 hours.