In the bowl of a food processor, pulse together the flour and salt until combined.

Add the cold butter and mix until it resembles a coarse meal.

With the machine running, slowly add the cold water and process until a dough comes together. Add more water, 1 teaspoon at a time, if the dough seems too dry.

Divide the dough into two equal-sized balls and flatten them into disks. Wrap each disk in plastic wrap and chill in the refrigerator for at least 1 hour before rolling out.

On a lightly floured surface, roll out one of the pie crust disks to fit your skillet. Refrigerate the other pie dough for another use, or freeze it.

Preheat your oven to 400°F (200°C).

In a large bowl, whisk together the cornstarch, 0.5 tablespoon cinnamon, nutmeg, 0.25 cup sugar, and 0.25 teaspoon salt until well combined.

Add the sliced apples to the bowl and toss to coat them evenly with the spice mixture. Set aside.

Melt 2 tablespoons of softened butter in a large oven-safe iron skillet over medium heat.

Stir in the thawed mixed berries and let them cook down for about 4 minutes, stirring occasionally.

Add the coated apples to the skillet with the berries and continue to cook for 2 more minutes, stirring gently.

Remove the skillet from the heat and let the fruit filling cool for a minute or two.

Carefully lay the rolled-out pie crust over the fruit filling in the skillet, tucking the sides in around the edges. You can crimp the edges for a decorative look if desired.

Brush the top of the pie crust with the beaten egg (egg wash).

In a small bowl, combine 0.5 teaspoon cinnamon and 0.5 teaspoon sugar. Sprinkle this cinnamon-sugar mixture evenly over the top of the crust.

Place the iron skillet in the preheated oven and bake for 25 to 30 minutes, or until the crust is golden brown and the fruit filling is bubbling.

Remove from the oven and let cool slightly before serving. Serve warm with a scoop of vanilla ice cream, if desired.