01 - Combine flour, salt, and sugar in a large bowl. Cut in cold butter using a pastry cutter or fingers until mixture resembles coarse crumbs. Add ice water 1 tablespoon at a time, mixing until dough just comes together. Divide dough in half, shape into disks, wrap in plastic, and refrigerate for at least 1 hour.
02 - Mix rhubarb, strawberries, sugar, cornstarch, salt, vanilla, and lemon juice in a large bowl. Toss gently to combine. Set aside for 15 minutes.
03 - Roll out one dough disk on a lightly floured surface to fit a 9-inch pie dish. Ease into the dish, leaving overhang.
04 - Spoon the filling and its juices into the crust. Dot with butter pieces.
05 - Roll out the second dough disk and cut into 1-inch strips. Lay half the strips parallel over the filling. Weave remaining strips in a lattice pattern. Trim excess and crimp edges to seal.
06 - Whisk egg and milk together. Brush over the lattice crust. Sprinkle with coarse sugar if desired.
07 - Preheat oven to 400°F. Place pie on a baking sheet. Bake for 20 minutes. Reduce oven to 350°F, cover loosely with foil, and bake for 35 minutes more until crust is golden and filling is bubbling.
08 - Let pie cool completely on a wire rack for at least 2 hours before slicing to allow the filling to set properly.