01 - Whisk flour, salt, and sugar in a large bowl. Cut in cold butter until mixture resembles coarse crumbs. Gradually add ice water, mixing just until dough comes together. Divide into two disks, wrap in plastic, and refrigerate for at least 1 hour.
02 - Preheat oven to 400°F.
03 - Combine strawberries, rhubarb, sugar, cornstarch, cinnamon, salt, lemon juice, and vanilla in a large bowl. Toss gently to coat. Let stand for 10 minutes.
04 - Roll out one dough disk to fit a 9-inch pie plate. Press into plate and trim excess dough.
05 - Spoon filling into the crust, spreading evenly.
06 - Roll out second dough disk and cut into 3/4-inch strips. Lay half the strips parallel over the filling, then weave remaining strips perpendicular to form a lattice pattern.
07 - Trim and crimp edges to seal. Brush lattice and edges with beaten egg. Sprinkle with coarse sugar if desired.
08 - Place pie on a baking sheet. Bake for 20 minutes at 400°F, then reduce heat to 350°F and bake for 35-40 minutes more, until crust is golden and filling is bubbling.
09 - Cool for at least 2 hours before slicing to allow filling to set properly.