In a large bowl, using a pastry cutter or fork, cut butter and shortening into the flour and salt until the largest clumps are coated in flour. The butter and shortening should be pea-sized. 

Add ice-cold water, a tablespoon or two at a time, and mix until a crumbly dough forms.

Knead dough together a few times to bring together and separate into two disks. 

Wrap each disk of dough in plastic wrap and chill for at least 3 hours.

When you are ready to assemble the pie, add the frozen berries, sugar, cornstarch and orange or lemon juice to a medium saucepan. Bring to a gentle simmer over medium heat, stirring occasionally. Allow the berries to simmer and thicken for 3-4 minutes then remove from heat and allow to cool to room temperature while you form the pie.

reheat oven to 375 degrees. Remove the dough from the refrigerator and place it between two large pieces of plastic wrap. Roll from the center, turning the dough as you go, until you have a 13 by 18 inch rectangle.

The dough will be thin. Remove the top piece of plastic wrap and flip the dough over onto a 10 by 15 inch jelly roll pan. Remove the second sheet of plastic wrap, and gently press the dough (without stretching it) into the pan. Roll the overhanging edges of the dough under and press them to the side of the pan.

Pour the filling evenly over the bottom crust

Roll out the remaining dough into a ten by 15-inch rectangle.

At this point, you can cut out designs or cover the entire pie with the top crust and crimp the edges.

In a small bowl, whisk egg and milk together. Brush the wash over the top crust and edges.

Bake the pie for 35-40 minutes or until the top is golden brown. Allow to cool and then dust with powdered sugar and cut into squares.