Preheat oven to 400.
In a large bowl, whisk together the cornstarch, cinnamon, nutmeg, sugar, and salt.
Add the apples and toss to coat. Set aside.
Melt the butter in a large iron skillet over medium heat.
Mix in the berries and let cook down for 4 minutes.
Stir in the apples and continue to cook for 2 more minutes.
Remove from heat and let cool for a minute or two.
Roll out the pie crust and lay over the fruit filling, tucking the sides in.
Brush the top of crust with egg wash.
Combine the cinnamon and sugar in a small bowl and sprinkle the top of crust with cinnamon sugar.
Place the iron skillet in the oven and bake for 25 to 30 minutes, or until crust is golden brown and fruit is bubbling.