Defrost or make a single pie crust. Roll it out, combine frozen fruit with sugar and cornstarch, plop the fruit on the pie crust and fold up the edges of the crust. Bake at 350 for roughly an hour.
Preheat your oven to 350, set your rack at the lowest one-third position
Roll out the pie crust and place in something smaller than a regular pie plate. A rectangular 9 x 7 dish works well
Mix the frozen berries, cornstarch, and sugar in a bowl.
Pile the berries in the pie dough and gently fold the perimeter of the dough up an over, making a "lid" with a hole in the middle. The crust should not cover the top of the pie completely.
Beat the egg. Paint the top of the pie crust with the egg wash and sprinkle with granulated sugar.
Bake at 350 for about an hour. You'll smell it before then, and that's when it's time to peek in the oven and make sure all is ok. Leave it in until the fruit is bubbling and the crust is a rich golden-brown color.
Notes
Fresh or frozen blueberries work. Feel free to mix up the fruit - you just need enough to fill the center of the pie dough. I've made this with a small container of fresh blueberries and 2 fresh pears.