Prepare the Crust: In a large bowl or food processor, pulse flour and salt. Cut in the cold butter until the mixture resembles coarse crumbs with some pea-sized pieces remaining. Gradually add ice water, 1 tablespoon at a time, mixing until the dough just comes together. Divide into two slightly unequal discs, wrap in plastic, and refrigerate for at least 1 hour.

Prepare the Filling: In a large bowl, gently combine the strawberries, rhubarb, granulated sugar, cornstarch, lemon zest, and lemon juice. Toss until the fruit is evenly coated and set aside while you roll out the dough.

Assemble the Pie: Preheat oven to 400°F (200°C). On a lightly floured surface, roll out the larger dough disc to a 12-inch circle and carefully place it into a 9-inch pie plate. Trim the edges, leaving about a 1-inch overhang. Pour the fruit filling evenly into the crust.

Top the Pie: Roll out the second dough disc. Either place it over the filling as a solid top crust (cutting slits for steam vents) or cut it into strips to create a lattice top. Crimp the top and bottom crusts together to seal. Brush the top crust lightly with the beaten egg wash and sprinkle with coarse sugar, if desired.

Bake the Pie: Place the pie on a baking sheet (to catch drips). Bake at 400°F (200°C) for 20 minutes. Reduce the oven temperature to 375°F (190°C) and continue baking for another 40-45 minutes, or until the crust is deep golden brown and the filling is bubbly. If the edges brown too quickly, cover them loosely with foil.

Cool and Serve: Let the pie cool completely on a wire rack for at least 3 to 4 hours before slicing and serving. This allows the filling to set properly.