01 - Combine flour, salt, and sugar in a large bowl. Add cold butter and cut in using a pastry cutter or fingers until mixture resembles coarse crumbs. Gradually add ice water, mixing just until dough comes together. Divide into two disks, wrap, and chill for at least 1 hour.
02 - Preheat oven to 400°F.
03 - Toss strawberries and rhubarb with sugar, cornstarch, vanilla, lemon juice, and salt in a large bowl. Set aside to macerate for 15 minutes.
04 - Roll out one dough disk on a lightly floured surface to fit a 9-inch pie dish. Line the dish with dough, trimming any excess.
05 - Fill the crust with strawberry-rhubarb mixture, spreading evenly.
06 - Roll out second dough disk and cut into 3/4-inch strips. Lay half strips horizontally over pie, then weave remaining strips vertically to form a lattice pattern.
07 - Trim excess dough and crimp edges to seal. Brush lattice and edges with beaten egg and sprinkle with coarse sugar if desired.
08 - Place pie on a baking sheet and bake at 400°F for 20 minutes. Reduce temperature to 350°F, cover edges with foil if browning too quickly, and bake for 30-35 minutes until crust is golden and filling is bubbling.
09 - Cool completely before slicing to allow filling to set properly.