01 - In a large bowl, combine flour, sugar, and salt. Cut in the cold cubed butter using a pastry blender or fork until the mixture forms coarse crumbs.
02 - Gradually add ice water, 1 tablespoon at a time, mixing just until the dough comes together. Divide dough in half, shape into discs, wrap, and refrigerate for at least 1 hour.
03 - Preheat oven to 400°F.
04 - In a large bowl, gently toss cherries and blueberries with sugar, cornstarch, lemon juice, vanilla extract, ground cinnamon, and a pinch of salt. Let the mixture rest for 10 minutes.
05 - On a lightly floured surface, roll out one disc of dough to fit a 9-inch pie dish. Transfer to the dish and trim excess dough from the edges.
06 - Pour the fruit filling evenly into the prepared pie crust.
07 - Roll out the second disc of dough and place over the filling, using either a lattice or solid crust. Trim and crimp the edges, and cut small slits if using a solid crust for steam to escape.
08 - Whisk the egg with milk. Brush the mixture over the top crust and sprinkle with coarse sugar if desired.
09 - Place the pie on the oven's lower rack and bake at 400°F for 20 minutes. Reduce the heat to 350°F and continue baking for 30 minutes, or until the crust is golden and the filling is bubbling.
10 - Allow the pie to cool completely before slicing for clean portions.