In a large mixing bowl, add in apple slices, blueberries, brown sugar, cornstarch, cinnamon, and vanilla extract. Fold together until the fruits are coated.
Take your pie dish out of the fridge and pour all of the fruit mixture into the pie crust. There should be enough fruit fill the dish and dome up out of the dish significantly.
Take the dough strips out of the fridge and begin arranging the lattice on top of the pie. Place down two of the longest strips in the middle of the pie in an X shape. Place two more strips on either side of the strip that is on the bottom of the X.
Lift up the strip that is on the bottom of the X, the “under” piece. Place a strip under the strip you lifted up but over the other two strips. Repeat lifting up all the “under” strips when placing a new strip, alternating with each new strip. See photos in the post - it’s easier to understand this by seeing it rather than reading it.
Once the lattice is done, trim off any overhanging ends of the strips. Fold the overhang of the bottom crust inwards, sealing off the lattice ends. Press the crust with a fork to remove some bulk.
Press your index finger and thumb on one hand against the inside of the crust while pressing your index finger on the other hand against the outside of the crust to crimp the crust into a wavy pattern.
Preheat the oven to 425°F.
Use a pastry brush to apply a thin layer of egg wash over all of the pie pastry. Sprinkle generously with sugar crystals or coarse sugar. Chill the pie in the fridge while the oven finishes preheating. This allows the butter to solidify again before baking to retain the shape of the crimped crust and for a flaky crust.
Place an empty tray on the bottom rack to catch any juices that bubble over. Bake for 15 minutes. Lower the heat to 350°F and continue baking for 50-55 minutes or until the crust is golden brown and filling is bubbling.
Cool on a cooling rack for at least 1 hour or until you can touch the bottom of the pie dish with your bare hand as the filling needs time to set.