Prepare the pie crust by combining flour, salt, and cold butter in a food processor until the mixture resembles coarse crumbs. Add cold water and mix until the dough just comes together. Divide the dough into two discs, wrap in plastic, and refrigerate for at least 30 minutes.
In a large bowl, gently toss the fresh blueberries with sugar, cornstarch, and a pinch of salt to coat the berries evenly.
Roll out one disc of the chilled pie dough and fit it into a 9-inch pie dish. Pour the blueberry filling into the prepared crust.
Roll out the remaining pie dough and cut into strips or shapes to create a lattice or decorative top crust. Arrange the dough strips or shapes over the filling, crimping the edges to seal.
Brush the top crust with beaten egg or milk and sprinkle with coarse sugar, if desired.
Bake the pie in a preheated oven until the crust is golden brown and the filling is bubbling, about 40-50 minutes. Allow the pie to cool completely before slicing and serving.

Triple Berry Trifle