Prepare the double pie crust as per my pie crust recipe. Make sure to divide the dough into two discs and chill both in the fridge for at least 1 hour. When the dough is ready to use, make the filling. In a large bowl, add blueberries, white sugar, lemon zest, lemon juice, ground cinnamon, salt, and cornstarch. Mix everything together well and set aside.
Lightly flour your bench and roll out one chilled pie disc into a 12-inch circle (make sure to keep the second crust in the fridge in the meantime—see note 2). If the dough is too firm (usually if the dough has been in the fridge longer than an hour), let it sit at room temperature for about 15 minutes before rolling it out.
Wrap the rolled-out pie dough around your rolling pin and gently unroll it over a 9-inch pie dish. Press the dough into the dish without stretching it, leaving some overhang around the edges.
Pour the blueberry filling into the crust and spread it out, slightly mounding it in the centre. Cut the unsalted butter into small cubes and dot it over the top of the filling.
Roll out the second disc of dough into an 11-inch circle and cut it into 1-inch-wide strips. Lay half of the strips vertically over the pie. Fold back every second strip halfway, then lay a new strip across the pie. Unfold the strips back over the top, then repeat with the remaining strips to create a lattice pattern (see video for demonstration).
Trim any excess dough hanging over the edge, leaving about ½ an inch hanging over the edges of the pie dish. Roll the edges inward to create a thick rim that sits just inside the pie dish. I like to flute the edges (this is optional). To do this, press your index finger from one hand on the inside of the crust, and pinch around it using your thumb and index finger from the other hand. Repeat around the whole pie (see video for demonstration).
Lightly beat an egg and brush it over the top crust. Don’t use too much—just enough to coat it. Sprinkle sugar on top, then place the pie in the fridge while you preheat the oven.
Place a tray in the lower third of your oven and preheat it to 375°F (190°C). The tray will act like a hot plate to help cook the bottom crust and catch any filling that might bubble over.
Once the pie crust feels firm, place the pie onto the hot tray in the oven. Bake for 60–80 minutes or until the filling is bubbling in the centre (see note 3) and the crust is golden brown. Check around the 45-minute mark and tent with foil if the top is browning too quickly.
Once baked, let the pie cool at room temperature for about 3 hours before slicing. This helps the filling set properly. Slice with a serrated knife and serve on its own or with whipped cream or vanilla ice cream.