In a small pot over medium heat, cook 4 cups of the frozen blueberries with 2 tablespoons of lemon juice for 20 minutes. Stir occasionally and use a large spoon to smash the berries against the side of the pot until most of the berries have broken down. Keep the remaining 3 cups of blueberries in the freezer for now.
7 cups frozen blueberries, 2 TBS lemon juice
Pour the cooked berries into a heat proof bowl, place in the fridge, and let them cool completely. This will take at least 30 minutes. You can do this step up to 3 days before baking day.
Set your oven to 400°F and line a baking tray with aluminum foil. Set aside.
In a small bowl, combine the sugar and lemon zest. Use your fingers to massage the zest into the sugar until fragrant and well combined.
½ cup sugar, 1 TBS lemon zest
In a large bowl, combine the remaining 3 cups of frozen blueberries with the lemon zest-sugar mixture, cornstarch, cinnamon, ginger, salt, and vanilla. Stir well, then add the completely cooled cooked blueberries and mix thoroughly. Place the mixture in the fridge while you prepare the pie plate.
3 TBS cornstarch, 1 tsp cinnamon, ½ tsp ginger, 1 tsp kosher salt, 1 tsp vanilla
Roll out your first piece of pie dough into a 12-inch-wide circle and fit it into a 9-inch pie plate. Roll out the second piece of pie dough and cut out six strips, each about 1 ½ inches wide.
2 9" pie crusts
Pour the blueberry filling into dough lined pie plate. Lay the dough strips over the filling, weaving them into a lattice pattern. Trim and crimp the edges. Brush with an egg wash and sprinkle with coarse sugar.
Place the pie on the prepared baking tray and bake for 25 minutes at 400°F on the center rack. Then, reduce the oven temperature to 350°F and bake for an additional 40 to 45 minutes. To prevent browning, check pie often and tent edges with foil when they are golden brown.
Let the pie cool at room temperature for at least 3 hours before slicing. This allows the filling to set properly.