Combine the blueberries, sugar and cornstarch, lemon zest, lemon juice, and sea salt in a large bowl and mix well. Lightly press down on the blueberries to break open a few and release their juices. Cover the bowl with a tea towel and let sit for 15 minutes.
Preheat the oven to 425F
Place a tray or foil sheet on the bottom rack of your oven and move the middle rack to the bottom third.
Add the cold water to an egg in a cup and mix with a fork. This is your egg wash.
Roll out bottom crust and place in pie plate. Trim the edges so that crust reaches the edge of the pie pan, and brush with the egg wash.
Roll out the remaining pastry dough for the top this time make the top larger than your pie pan - so for a 9.5" pan like I'm using, roll out to 12-14"
Pour the filling into the bottom crust and spread evenly, and dot the pie with the diced butter.
Roll the top crust around your rolling pin – like in the picture shown here and then centre it on your pie. Gently turn the edges under – you may need to do some trimming and then flute the edges.
Brush it with the egg wash and return to the refrigerator for another 20 minutes.
Bake in a 425F oven for 25 minutes in the bottom third of the oven.
Reduce heat to 375F and bake another 40-50 minutes until the juices are bubbling in the centre and the pie crust is golden.
Cool completely - at least 8 hours (or overnight) before slicing.