Preheat the oven to 425°F with a rack in the middle position.
Whisk the flour and salt together in a large bowl. Cut in the cold butter with a pastry blender or two knives (or pulse briefly in a food processor) until the butter breaks down into pea-sized pieces.
Add the cold water a little at a time, mixing just until the dough comes together — you may not need all of it, or may need a touch more.
Divide the dough into two equal discs, wrap each in plastic, and refrigerate for at least 1 hour, or overnight.
Let the dough sit out for a couple of minutes to soften slightly, then roll one disc out on a floured surface into a circle large enough to overhang your pie plate by about an inch. Ease it into the dish and shape the edge however you like.
Toss the blueberries, sugar, and flour together, then fill the crust with the mixture.
Roll out the second disc and either lay it whole over the top (cutting a few vents for steam) or cut it into strips or shapes to arrange over the filling. There are many ways to make the top of a blueberry pie.
Brush the top crust with the beaten egg and sprinkle generously with coarse sugar.
Bake at 425°F for 20 minutes, then reduce the heat to 350°F and continue baking for another 45–55 minutes, until the crust is golden and the filling bubbles. Tent with foil if the crust browns too fast, and consider placing a baking sheet on the rack below to catch any drips.
Cool on a wire rack before slicing. Best served warm.
