Whisk together flour, salt, and sugar in a large bowl until evenly combined.

Add cold, cubed butter to the flour mixture and use a pastry cutter or fork to cut into pea-sized pieces.

Sprinkle 6 tablespoons of ice water over the mixture and stir gently with a fork until the dough just begins to come together, adding more water if needed.

Divide the dough in half and shape each into a disk, then wrap in plastic wrap and chill in the refrigerator for at least 30 minutes.

Preheat the oven to 425°F (220°C) and lightly flour your work surface.

Roll out one dough disk to a 12-inch circle, about 1/8-inch thick, and transfer to a 9-inch pie dish, gently pressing into the corners.

Pour your prepared chicken pot pie filling into the crust, ensuring it is slightly cooled to avoid warming the dough.

Roll out the second dough disk and place over the filling, trim excess dough, and crimp the edges to seal.

Cut four small slits in the top crust to allow steam to escape.

Whisk together egg and water, then brush the mixture over the top crust and sprinkle with coarse sea salt if desired.

Chill the assembled pie in the refrigerator for 20 minutes before baking.

Bake the pie on a baking sheet for 35 to 40 minutes until the crust is deep golden brown and the filling bubbles around the edges, tenting with foil if the crust browns too quickly.