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