01 - Combine flour, sugar, and salt in a large mixing bowl. Cut in cold, cubed butter using a pastry blender or fork until the texture resembles coarse crumbs. Add ice water one tablespoon at a time, mixing until dough just comes together. Divide dough in half, flatten into discs, wrap in plastic, and refrigerate at least 1 hour.
02 - Set oven rack to lower third and preheat oven to 400°F.
03 - In a large bowl, toss cherries and blueberries with granulated sugar, cornstarch, lemon juice, vanilla extract, cinnamon, and a pinch of salt. Allow mixture to rest for 10 minutes.
04 - Roll out one dough disc on a floured surface to fit a 9-inch pie dish. Drape crust over dish, gently pressing to fit. Trim excess dough from the edges.
05 - Pour prepared fruit filling into the lined dish. Roll out the second dough disc and cover pie using lattice or solid crust. Seal and crimp the edges. For solid-top crust, cut several small vents to release steam.
06 - Whisk egg and milk together. Brush mixture evenly over the top crust and sprinkle with coarse sugar if desired.
07 - Place pie on the lower oven rack and bake for 20 minutes. Reduce oven temperature to 350°F and continue baking 30 minutes more until crust is deep golden and filling is bubbling.
08 - Remove from oven and cool completely on a wire rack before slicing to allow the filling to set.