01 - Combine flour, sugar, and salt in a 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, until the dough just comes together. Divide the dough in half, shape into two discs, wrap in plastic, and refrigerate for at least 1 hour.
02 - Preheat the oven to 400°F (200°C).
03 - In a large bowl, gently mix together cherries, blueberries, sugar, cornstarch, lemon juice, vanilla extract, cinnamon, and a pinch of salt. Let the mixture rest for 10 minutes.
04 - Roll out one dough disc on a lightly floured surface to fit a 9-inch pie dish. Place the dough in the dish and trim the excess edges.
05 - Pour the fruit filling into the prepared pie crust. Roll out the second dough disc and lay it over the filling, creating a lattice or solid crust as desired. Trim and crimp the edges. If using a solid top, cut a few small slits to allow steam to escape.
06 - Whisk the egg with milk, then brush the mixture over the top crust. Sprinkle with coarse sugar if desired.
07 - Place the pie on the lower oven rack and bake for 20 minutes. Lower the oven temperature to 350°F (175°C) and continue baking for 30 minutes, or until the crust is golden brown and the filling is bubbling.
08 - Allow the pie to cool completely before slicing and serving.