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Preheat the oven to 450°F and adjust the oven rack to the center position. Roll bottom pie crust out and fit inside a 9-inch pie plate. Add potato to bottom and spread into a smooth, even layer. Using a large spoon, make 5 indentations in the potatoes radially. Fill each indentation with gravy.
Roll out top crust and seal pie. Whisk egg white with 1 tablespoon water. Brush top crust with egg white mixture and cut 3 to 5 holes for ventilation. Sprinkle sparingly with coarse sea salt. Place pie in oven and bake for 15 minutes until pale golden brown. Reduce heat to 350°F and continue baking until deep golden brown and mixture is starting to bubble through the vents, about 45 minutes longer. Allow to cool 15 minutes before serving.