Make the crust: Prepare a full batch of my gluten-free pie crust. Cut the dough into two and wrap individually. Refrigerate until ready to use.
Preheat your oven to 425°F (220°C).
Prepare the pie crust: Take out one of the pie dough disks from the refrigerator. Use a rolling pin to press down on the pie crust disk a few times to soften it. Dust your rolling pin and a piece of parchment paper with gluten-free flour. Roll out the bottom crust into a large circle onto the piece of parchment paper, until it is about ¼ inch thick and 12 inches in diameter for a 9-inch pie dish.
Place the pie dish upside down on the center of the rolled-out dough. Using the parchment paper for support, carefully flip the dough and pie dish over so the dough lines the inside of the dish. You want to have about 1.5 inches of overhang on all sides. Take the overhang and roll it under itself to create a thick edge.
Place the crust in the refrigerator to chill while you prepare the filling and topping.
Make the blueberry filling: In a large bowl, combine the blueberries, sugar, cornstarch, lemon juice, lemon zest, cinnamon, nutmeg, salt and vanilla extract. Gently toss to combine and set aside.
Remove the pie crust from the refrigerator. Pour the blueberry pie filling into the pie crust, spreading it out evenly.
Make the lattice topping: Roll out the second disk of dough into a large circle, about 12-inches in diameter. Cut it into 1-inch strips using a pizza cutter or a knife. Lay half of the strips horizontally over the pie. Fold back every other strip halfway and lay one strip perpendicular. Unfold the strips over the perpendicular strip and repeat with the remaining strips to create a lattice pattern.
Tuck the excess dough from the bottom crust and the strips underneath themselves to create a thick lip around the pie. Crimp the edges decoratively using your fingers or a fork.
Egg wash: In a small bowl, beat the egg with the water to create the egg wash. Brush the egg wash over the lattice crust and the edges of the pie.
Dot the openings between the lattice with the butter and sprinkle the top with the coarse sugar.
Bake the pie: Place the pie on a baking sheet to catch any drips. Bake at 425°F (220°C) for 15 minutes. Reduce the oven temperature to 375°F (190°C) and continue baking for 45 minutes to 1 hour, or until the crust is golden brown and the filling is bubbling. If the pie is browning too quickly, place aluminum foil over the top of the pie to protect the lattice topping and edges from burning.
Remove the pie from the oven and let the pie cool on a wire rack for at least 2 hours before serving. Cooling completely allows the cherry filling to set and thicken, making slicing easier and ensuring that the filling doesn’t pour out of each slice of pie.
Slice and serve!