01 - In a large mixing bowl, blend flour, sugar, and salt evenly.
02 - Add cold, cubed butter and use a pastry blender or fingers to work it into the dry ingredients until the mixture is a coarse crumble.
03 - Gradually add ice water, one tablespoon at a time, mixing until the dough begins to come together. Avoid overworking.
04 - Divide the dough into two even portions, shape each into a disc, wrap with plastic, and refrigerate at least 1 hour.
05 - Set the oven to 400°F.
06 - In a large bowl, gently combine cherries, blueberries, sugar, cornstarch, lemon juice, vanilla extract, cinnamon, and a pinch of salt. Let mixture stand for 10 minutes.
07 - On a lightly floured work surface, roll one chilled dough disc into a round large enough for a 9-inch pie dish. Line the dish and trim excess.
08 - Spoon the prepared fruit filling into the crust-lined pie dish, distributing evenly.
09 - Roll the second dough disc and place over the filling, either as a lattice or solid. If solid, cut a few small vents. Trim and crimp the edges for a secure seal.
10 - Whisk the egg together with milk. Brush mixture over the top crust, then sprinkle with coarse sugar if desired.
11 - Place pie on the lower oven rack and bake at 400°F for 20 minutes.
12 - Decrease oven temperature to 350°F and bake for another 30 minutes, or until the crust is crisp and golden, and filling is visibly bubbling.
13 - Transfer the baked pie to a rack and cool completely before slicing and serving.