01 - In a large mixing bowl, combine all-purpose flour, sugar, and salt. Cut in cold, cubed butter using a pastry blender or fork until mixture forms coarse crumbs. Gradually drizzle in ice water, 1 tablespoon at a time, mixing until dough comes together. Divide into two equal discs, wrap in plastic, and chill for at least 1 hour.
02 - Set oven rack in the lower third and preheat oven to 400°F (200°C).
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. Allow mixture to stand for 10 minutes to macerate the fruit.
04 - Roll out one chilled dough disc on a lightly floured surface to fit a 9-inch pie dish. Transfer the dough to the dish, gently pressing into corners, and trim any excess overhang.
05 - Spoon the prepared cherry and blueberry filling into the lined pie dish, distributing evenly.
06 - Roll out the second dough disc for the top of the pie. Cover the filling with a solid or lattice crust, trimming and crimping edges to seal. If using a solid crust, cut a few small vents for steam.
07 - Whisk together beaten egg and milk. Brush mixture over the top crust. Sprinkle coarse sugar for added crunch and shine, if desired.
08 - Place pie on the lower oven rack. Bake at 400°F for 20 minutes. Reduce oven temperature to 350°F (175°C) and continue baking for 30 minutes, until crust is golden and filling is bubbling.
09 - Remove baked pie from the oven. Allow to cool completely before slicing and serving for clean portions.