01 - In a large mixing bowl, combine flour, sugar, and salt. Cut in the cold butter with a pastry blender or fork until the mixture forms coarse crumbs. Gradually add ice water, one tablespoon at a time, mixing just until the dough begins to come together. Divide the dough in half, shape into discs, wrap tightly, and chill in the refrigerator for at least one hour.
02 - Set the oven to 400°F (200°C) and position a rack in the lower third.
03 - In a large bowl, gently toss the pitted cherries and blueberries with granulated sugar, cornstarch, lemon juice, vanilla extract, ground cinnamon, and a pinch of salt. Let the mixture rest for 10 minutes to allow the juices to develop.
04 - On a lightly floured surface, roll one disc of dough into a circle large enough to fit a 9-inch pie dish. Transfer the dough to the dish, gently easing it into the corners and trimming any excess overhang.
05 - Pour the prepared fruit mixture evenly into the pie shell. Roll out the second disc of dough and place it over the filling, using either a lattice design or a solid crust. Trim and crimp the edges to seal, and cut several small slits in the top crust if using a solid sheet.
06 - Whisk the egg with the milk. Brush the top crust with the egg wash and sprinkle with coarse sugar if desired.
07 - Place the pie on the lower oven rack and bake at 400°F (200°C) for 20 minutes. Reduce oven temperature to 350°F (175°C) and continue baking for 30 additional minutes, or until the crust is golden brown and the filling is bubbling.
08 - Allow the pie to cool completely on a wire rack before slicing to ensure the filling sets properly.