Peel, core, and slice the Granny Smith and Honeycrisp apples into ¼-inch slices. Cut those slices in half so you have smaller, bite-size pieces of apples. Place the apples into a large mixing bowl.
Add the light brown sugar, granulated sugar, apple pie spice blend, salt, and lemon juice to the apple slices. Stir to evenly coat all the apple slices in the sugar and spices. Set aside for 1 hour to allow the juices to come out of the apples.
Preheat the oven to 400°F. Place a 9-inch pie plate on top of a parchment-lined rimmed baking tray. This will make sure that any juices that may bubble out from your pie do not spill into your oven and create a mess.
While your oven is preheating, thaw and unroll both your pie crusts according to the package instructions.
Place one of the unrolled pie crusts into the pie plate.
Using a slotted spoon, transfer the apples into the pie crust. Spread them out into an even layer. Do not pack them in tight.
Add the cornstarch to the bowl with the remaining juices and whisk to create a slurry. You do not want any lumps of cornstarch remaining. Pour the slurry evenly over the apples in the pie pan.
Randomly place the little pieces of unsalted butter cubes over the top of the apples.
Using a sharp paring knife, cut 1½-inch strips from the second unrolled pie crust. You will create a lattice topping and crimp the edges of the crust to seal.
In a small bowl, whisk together the egg and water to create an egg wash. Using a pastry brush, brush the egg wash over the lattice-topped apple pie.
Sprinkle the coarse sanding sugar over the top of the lattice crust. The egg wash will help keep the sanding sugar adhered to the crust. This is an optional step but it does create a beautiful crust with a great little crunch to it.
Bake for 30 minutes. At this point, you may want to tent your apple pie with aluminum foil to prevent your crust from over-browning. Bake for an additional 15 to 20 minutes.
Allow your apple pie to cool completely before slicing and serving.