Preheat oven to 350º.
Pie Crust: In a large bowl, whisk together the flour and salt. Add the cold butter and use a pastry cutter into the flour until combined and the size of peas. Drizzle 2 tbs. of ice cold water at a time and use a spatula to stir until the dough holds together. Divide and form into 2 balls. Wrap in plastic wrap and chill for 2 hours. Roll out on a lightly floured surface to 1/4 inch thinkness. Make sure you have at least 1 inch of over hang.
Filling: Mix together the brown sugar, flour, cinnamon and nutmeg. Place the apple slices in a large mixing bowl. Toss with the dry ingredients mixture to coat and set aside.
To Carmelize Crust: Melt butter in a 10-inch cast-iron skillet over medium heat; add brown sugar, and cook, stirring constantly, 1 to 2 minutes or until sugar is dissolved. Remove from heat, and place piecrust in skillet over brown sugar mixture. Spoon apple mixture over piecrust.
Topping: Mix flour, brown sugar and butter until crumbly. Add topping working from the outside inward.
Bake for 60-70 minutes or until golden brown and bubbly, shielding with aluminum foil during last 10 minutes to prevent excessive browning, if necessary. Cool on a wire rack for at least 30 minutes.