01 - In a large bowl, combine flour, sugar, and salt. Cut in cold butter until mixture resembles coarse crumbs. Gradually add ice water, 1 tablespoon at a time, mixing until dough just comes together. Divide dough in half, shape into discs, wrap in plastic, and chill for at least 1 hour.
02 - In a large bowl, combine strawberries, rhubarb, sugar, cornstarch, lemon juice, vanilla, and salt. Toss gently and set aside for 10 minutes.
03 - Preheat oven to 400°F. On a floured surface, roll out one dough disc into a 12-inch circle. Line a 9-inch pie dish with the crust, letting excess hang over the edges. Pour filling into the crust, spreading evenly. Roll out the second dough disc and cut into 3/4-inch strips. Lay half the strips horizontally over the pie, then weave remaining strips vertically to create a lattice. Trim excess dough and crimp edges to seal. Brush lattice and edges with beaten egg and sprinkle with coarse sugar.
04 - Place pie on a baking sheet to catch drips. 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. Cool pie on a wire rack for at least 2 hours before slicing.