Place a rack in the center of the oven then preheat to 375 degrees. Lightly coat a jelly roll pan (approx. 10" x 15") with cooking spray then set aside.
Place 1 lb blueberries and ¾ cup granulated sugar in a medium saucepan. Over medium heat, stir until sugar dissolves and berries are thawed.
In a small bowl combine 3 Tbsp of cornstarch and 3 Tbsp of water. Pour into saucepan and bring to a boil. When mixture is clear and thick, remove from heat and transfer to a large bowl. Add remaining berries, zest and juice of lemon, 1 tsp cinnamon and ½ tsp salt. Mix well then set aside while you roll out the dough. NOTE: When working with fresh fruit, the amount of thickener you need depends on how ripe the fruit is. If you’re using super sweet, ripe fruit, increase the cornstarch by 1 or 2 Tbsp.
On a lightly floured surface, roll one of the pie crusts (if one half is slightly larger than the other; roll out the larger one first). Roll dough out into a rectangle approx. 18" x 13" in size. Then, roll dough around the rolling pin then transfer to your prepared jellyroll pan. Line the entire bottom and sides of the pan then trim away any excess overhang.
Pour blueberry mixture over the crust, spreading evenly. Roll the top crust into a 16" x 11" rectangle, large enough to cover the top of the pie, or cut strips of dough to make a lattice top. Tuck the edges of the top crust between the edge of the bottom crust and the pan, pinching the edges to seal the filling inside. Brush entire top crust with beaten egg then sprinkle with a light coating of granulated sugar.
Bake 40–45 minutes, until crust is golden brown on top and the filling is bubbly. Remove pie from the oven and cool on a rack until it achieves room temperature (or slightly warmer).
One Last Thing
Don’t hesitate to substitute the frozen berries with fresh berries. The recipe will work just as well.