01 - In a large mixing bowl, combine all-purpose flour, granulated sugar, and salt. Cut in cold unsalted butter using a pastry blender or fork until the mixture forms coarse crumbs. Gradually incorporate ice water, 1 tablespoon at a time, until dough just comes together. Divide dough into two equal portions, shape into discs, wrap, and refrigerate for at least 1 hour.
02 - Set oven temperature to 400°F (200°C) and allow to fully preheat.
03 - In a large bowl, gently toss cherries and blueberries with granulated sugar, cornstarch, lemon juice, vanilla extract, ground cinnamon, and a pinch of salt. Let sit for 10 minutes to allow juices to release.
04 - On a lightly floured surface, roll out one chilled dough disc into a round large enough for a 9-inch pie dish. Transfer to the dish and trim any overhang. Evenly distribute the prepared berry filling over the base crust.
05 - Roll out the second dough disc. Place over filling to create a lattice or full cover as desired. Trim and crimp edges to seal. If using a solid cover, cut a few small slits in the top to allow steam to escape.
06 - Whisk egg with milk. Brush over the surface of the top crust, then sprinkle with coarse sugar if desired for additional texture.
07 - Place pie on lower oven rack and bake for 20 minutes. Reduce oven temperature to 350°F (175°C) and continue baking for 30 minutes, or until crust is golden brown and filling is bubbling.
08 - Allow pie to cool completely on a wire rack before slicing and serving to ensure the filling sets.