Skillet Apple Berry Pie

For the pie crust (makes 2 pie crusts, but you will only need 1 for this recipe):
In the bowl of a food processor, pulse together flour and salt. Add butter and mix until it resembles a coarse meal. With the machine running, slowly add water and process until a dough comes together. Add more water if dough seems too dry.
Divide the dough into 2 equal size balls and flatten into disks. Wrap each disk separately in plastic wrap and chill at least 1 hour before rolling out.
For the filling:
Preheat oven to 400u0b0F.
In a large bowl, whisk together the cornstarch, salt, cinnamon, nutmeg, and sugar. Add the apples and toss to coat. Set aside.
Melt the butter in a large (8 or 10-inch size) cast iron skillet over medium heat. Mix in the berries and let them cook down for 4 minutes. Stir in the apple mixture and continue to cook for 2 more minutes. Remove from heat and let cool for a minute or two.
Put the pie crust onto a lightly floured surface. Roll out the pie crust so that it's large enough to cover the top of your skillet. Lay the crust over the fruit filling, tucking the sides in. Brush the top of the crust with the beaten egg.
For the cinnamon sugar topping, combine the cinnamon and sugar in a small bowl and sprinkle on top of the crust.
Place the iron skillet in the oven and bake for 25 to 30 minutes, or until crust is golden brown and fruit is bubbling.