01 - In a large mixing bowl, combine the all-purpose flour, granulated sugar, and salt. Using a pastry blender or fork, work in the cold, cubed unsalted butter until the mixture forms coarse crumbs. Gradually add ice water, one tablespoon at a time, mixing until the dough just comes together. Divide the dough in half, shape into discs, wrap tightly, and refrigerate for at least 1 hour.
02 - Preheat the oven to 400°F (200°C) and position a rack in the lower third of the oven.
03 - In a large mixing bowl, gently toss the cherries and blueberries with granulated sugar, cornstarch, lemon juice, vanilla extract, ground cinnamon, and a pinch of salt. Allow the mixture to sit for 10 minutes to macerate.
04 - Roll out one dough disc on a lightly floured surface into a circle large enough for a 9-inch pie dish. Place the rolled dough into the pie dish, pressing gently into the edges and trimming any excess overhang.
05 - Pour the prepared berry filling into the lined pie dish, spreading it evenly over the bottom crust.
06 - Roll out the remaining dough disc and gently lay it over the filled pie, creating a lattice or solid top as desired. Trim and crimp the edges to seal. If using a solid crust, make a few small slits in the top to allow steam to escape.
07 - Whisk the egg with milk and brush the mixture evenly over the surface of the top crust. Sprinkle with coarse sugar if desired for added texture.
08 - Place the pie on the lower oven rack and bake at 400°F for 20 minutes. Reduce the oven temperature to 350°F (175°C) and continue baking for 30 minutes longer, or until the crust is golden and the fruit filling is bubbling.
09 - Allow the pie to cool completely on a wire rack before slicing and serving to ensure the filling sets properly.