Peel and thinly slice the apples (keep the apples in a bowl of lemon water as you go to keep them from browning).
In a large bowl, toss the apples with the brown sugar, salt, cinnamon, lemon juice, and half of the cornstarch. Once the apples are well coated, let sit and macerate for 30 minutes, stirring occasionally.
Preheat the oven to 400˚F (200˚C).
Transfer the apples to a colander set over a medium bowl and let drain for about 15 minutes, until all of the liquid is drawn out.
Transfer the liquid released from the apples to a small pot over low heat.
In a small bowl, mix the rest of the cornstarch and the water to make a slurry. Add the slurry to the apple liquid and quickly stir to incorporate. Bring to a boil, then add the butter and stir until melted. Immediately remove from the heat and pour over the apples, stirring to coat.
Gently lay 1 rolled-out pie crust in a 10-inch (25-cm) pie dish.
Lay the apples in the pie crust, making sure they are flat and facing the rounded edges out in order to fit as many apples as possible in the crust. Pour any leftover liquid from the bowl over the apples.
Top with the other rolled-out pie crust. Trim the excess dough from the edges.
Press the 2 crusts together to seal, then fold the edges under. Crimp the edges.
Then brush all over with egg wash and sprinkle the with sanding sugar.
Use a paring knife to cut a few vents in the top for steam to escape.
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