01 - Combine all-purpose flour, sugar, and salt in a large mixing bowl. Cut in the cold butter using a pastry blender or fork until the mixture forms coarse crumbs. Gradually add ice water, one tablespoon at a time, mixing gently until dough just comes together. Divide dough in half, shape into discs, wrap in plastic, and refrigerate for at least one hour.
02 - Preheat the oven to 400°F (200°C).
03 - In a large bowl, gently toss pitted 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.
04 - On a lightly floured surface, roll out one disc of dough to cover a 9-inch pie dish. Carefully transfer dough to the dish, trim excess, and press into place. Pour prepared fruit filling into crust, spreading evenly.
05 - Roll out the second dough disc to cover the pie or create a lattice. Place over filling, trim edges, and crimp to seal. If using a solid crust, cut several small slits in the top for steam to escape.
06 - Whisk together beaten egg and milk. Brush mixture evenly over the top crust. Sprinkle with coarse sugar for added texture, if desired.
07 - Place pie on the lower oven rack. Bake for 20 minutes at 400°F (200°C). Reduce oven temperature to 350°F (175°C) and continue baking for 30 minutes, until the crust is golden and fruit filling is bubbling.
08 - Allow pie to cool completely before slicing and serving for optimal texture.