01 - In a large mixing bowl, combine flour, sugar, and salt. Work in the cold butter using a pastry blender or fork until the mixture forms coarse crumbs. Gradually incorporate ice water, one tablespoon at a time, just until the dough comes together. Divide dough into two equal portions, shape into discs, wrap tightly, and refrigerate for at least one hour.
02 - Set oven to 400°F (204°C) to ensure even baking.
03 - Gently toss cherries and blueberries with granulated sugar, cornstarch, lemon juice, vanilla extract, ground cinnamon, and a pinch of salt. Allow the fruit mixture to rest for 10 minutes to develop flavors.
04 - Roll out one disc of chilled dough on a lightly floured surface to fit a 9-inch pie dish. Press the dough into the dish and trim excess pastry around the edge. Pour the prepared fruit filling into the pastry-lined dish.
05 - Roll out the second pastry disc. Place over the filled pie, either as a lattice or solid top. Trim and crimp edges securely. If using a solid sheet, cut several small slits in the surface to allow steam to escape.
06 - Combine beaten egg with milk. Brush the mixture over the top crust for a glossy finish. Sprinkle with coarse sugar for added texture, if desired.
07 - Place the pie on the lower oven rack and bake for 20 minutes at 400°F (204°C). Reduce oven temperature to 350°F (177°C) and continue baking for 30 minutes, or until the crust is golden brown and the filling bubbles at the edges.
08 - Transfer the baked pie to a wire rack and cool completely to allow the filling to set before slicing and serving.