For the pie crust: In a large bowl, whisk together flour, sugar, and salt. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

Gradually add ice water, 1 tablespoon at a time, mixing until the dough just comes together. Do not overmix. Divide the dough in half, flatten each half into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes.

For the filling: In a large bowl, gently combine the sliced strawberries and chopped rhubarb. In a separate small bowl, whisk together the granulated sugar, cornstarch, lemon zest, and lemon juice. Pour this mixture over the fruit and toss gently to coat.

Preheat oven to 400°F (200°C). On a lightly floured surface, roll out one disk of dough into a 12-inch circle. Carefully transfer it to a 9-inch pie plate. Trim the edges, leaving about a 1-inch overhang.

Pour the strawberry-rhubarb filling into the prepared pie crust. Roll out the second disk of dough. You can either place it as a solid top crust (cutting vents) or cut it into strips for a lattice top.

Place the top crust over the filling. Crimp the edges to seal. Brush the top crust with the beaten egg wash and sprinkle with turbinado sugar.

Bake for 20 minutes at 400°F (200°C). Reduce oven temperature to 375°F (190°C) and continue baking for another 30-35 minutes, or until the crust is golden brown and the filling is bubbly. If the edges start browning too quickly, cover them with foil.

Let the pie cool completely on a wire rack for at least 2-3 hours before slicing and serving. This allows the filling to set properly.