01 - In a large mixing bowl, whisk together all-purpose flour, sugar, and salt.
02 - Cut cold, cubed butter into the flour mixture using a pastry blender or fork until the mixture forms coarse crumbs.
03 - Gradually add ice water, 1 tablespoon at a time, mixing until dough just holds together. Divide dough in half, shape into discs, wrap in plastic, and refrigerate for at least 1 hour.
04 - Preheat oven to 400°F.
05 - Gently toss cherries and blueberries with granulated sugar, cornstarch, lemon juice, vanilla extract, cinnamon, and a pinch of salt in a mixing bowl. Allow mixture to sit for 10 minutes.
06 - On a floured surface, roll out one disc of dough to fit a 9-inch pie dish. Lay the dough into the dish and trim excess edges.
07 - Pour fruit filling into the prepared crust.
08 - Roll out remaining dough and drape over the filling, or arrange strips for a lattice. Trim and crimp edges; cut several small slits in the top crust if not using a lattice.
09 - Whisk egg with milk. Brush over the top crust, then sprinkle coarse sugar if desired.
10 - Place the pie on the lower oven rack. Bake for 20 minutes at 400°F, then reduce oven temperature to 350°F and bake for an additional 30 minutes, or until the crust is golden and filling is bubbling.
11 - Allow pie to cool completely on a wire rack before slicing and serving.