In a small bowl, make a slurry with the cornstarch and water, and mix until there are no lumps.
In a separate bowl, toss together the apples, brown sugar, cinnamon and salt.
In a large skillet, melt the butter. Add the apples, and sauté until the apples are just starting to get tender and the sugar is all dissolved.
Add the slurry to the apples, and let simmer until the liquid is thick and clear, approximately 2-3 minutes.
Pour the filling onto a sheet pan to cool.
While the filling is cooling, roll out your crust.
Crust
Preheat oven to 375 degrees.
Roll out your bottom crust, and put it in your pie plate, leaving about a half-inch overhang all the way round. Place in the refrigerator until needed.
Roll out your top crust to approx a 12-inch circle, place on a parchment-lined sheet tray and refrigerate until needed.
Once the filling is cooled to room temperature, fill your bottom crust and cover with your pre-rolled top. Crimp and flute the edges, and cut some steam vents in the top.
Brush the top of your pie lightly with heavy cream or spritz with a little water, and sprinkle with granulated sugar or cinnamon sugar.
Bake approximately 40-45 minutes, until the top is golden and the filling is set.