Prepare your crust: Remove pie crusts from refrigerator about 15 minutes before using to soften slightly. Preheat your oven to 375°F (190°C).
Mix apple filling: In a large bowl, combine the sliced apples, granulated sugar, flour, cinnamon, nutmeg, salt, and lemon juice. Toss gently until apples are evenly coated.
Prepare the skillet base: Melt 4 tablespoons of butter in your cast iron skillet over medium heat. Add brown sugar and stir until dissolved, about 2 minutes, creating a caramel-like mixture.
Cook the apples: Remove skillet from heat and carefully arrange apple mixture in the skillet on top of the caramel mixture. The apples will be piled high but will cook down.
Add bottom crust: Roll out one pie crust and carefully place it over the apples, tucking the edges down between the apples and the side of the skillet. Cut 4-5 small slits in the center for steam to escape.
Create egg wash: Beat the egg with 1 tablespoon of water to create an egg wash. Brush the top crust with the egg wash and sprinkle with coarse sugar if using.
Bake to golden perfection: Place the skillet on a baking sheet (to catch any potential drips) and bake for 40-45 minutes until the crust is golden brown and the filling is bubbling through the slits.
Cool before serving: Allow the pie to cool for at least 30 minutes before serving to let the filling set. Serve warm with vanilla ice cream for the ultimate experience.