Preheat your oven to 425℉/218℃, and set the rack you’ll be baking the pie on in the lower third of the oven.
Make the filling: Add the blueberries, granulated sugar, cornstarch/flour, cinnamon, and lemon juice to a mixing bowl. Stir these ingredients together until all of the blueberries are coated and the cornstarch/flour mixture no longer appears dry. Set this aside for now.
Assemble the bottom crust for the pie: Take one pie crust out of your fridge (leave the other one in the fridge) and place it on a flat floured surface. Flatten the dough with a rolling pin in a circular manner until it is 12 inches in diameter. Carefully lift the crust off the flat surface and place it into either an 8-inch or 9-inch (2 inches deep) pie dish. Gently press the crust into the dish with your hands so that it is smooth and in line with the dish. Then use a knife to trim off any excess pie crust that might be hanging off the dish (you want no more than ¾ of an inch hanging off).
Pour the blueberry filling into the pie crust we just prepared.
Make the lattice top: Place the other pie crust on a flat floured surface. Roll it out to the same size and shape as the last one (circular, 12-inch diameter). Use either a sharp knife or a pizza cutter to cut the crust into 10 strips. Arrange 5 of the strips parallel over the top of the filled pie with gaps in between. Carefully fold back the 2nd and 4th strips and place 1 strip of crust perpendicular to them. Unfold the 2nd and 4th strips back over this new strip. Then fold back the 1st, 3rd, and 5th strips and place a second strip of crust perpendicular. Unfold those back then repeat these steps until the 3 remaining strips are placed into the lattice. If you’d like more information on how to do this process I recommend you check out the link provided in the “Pie Crust” section above. After the lattice is laid down press the edges of the strips to the bottom pie crust in order to seal them together. Then cut off any excess crust and either flute or crimp the edges with a fork or your fingers.
Cut the tablespoon of unsalted butter into small chunks then dot them over the holes left from the lattice crust.
Whisk together the egg and cream to make the egg wash. Then brush the lattice crust with the wash and sprinkle with coarse sugar.
(Optional) Place the pie in your fridge to let it cool for 20 minutes. You can skip this step but it does result in a better crust.
Bake: Place the pie on a baking sheet (that way it will catch any filling that might drip out while baking), and into your heated oven. Let the pie bake undisturbed for 20 minutes. Then lower the heat of your oven to 375℉/190℃ and let the pie continue to bake for an additional 40 to 50 minutes. And while it’s not necessary I highly recommend you add a pie shield or better yet full sheet of aluminum foil (check “Pie Crust” above) at this point so that the crust doesn’t burn. You will know that the pie is done when the crust is golden brown, and the filling is hot & bubbling with an internal temperature of 200℉/93℃.
Rest: Remove the baked pie from your oven and let it rest on a rack for roughly 4 to 5 hours. You want to make sure that it’s at least room temperature in order for the filling to set. Or else it will be too liquidy and run out when you slice into it. And trust me you don’t want to put in all this work to have that happen, so patience!
Once the blueberry pie is at room temperature slice it into pieces and enjoy!