**Make the Pie Crust:** In a large bowl, whisk together the flour 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 into a disc, wrap in plastic, and refrigerate for at least 30 minutes.

**Prepare the Filling:** In a large bowl, gently combine the sliced strawberries and rhubarb. Add the granulated sugar, 1/4 cup flour (or cornstarch), vanilla extract, and optional cinnamon. Toss gently until the fruit is evenly coated. Let sit for 15-20 minutes to allow the juices to release.

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

Pour the fruit filling into the pie crust, mounding it slightly in the center. Dot the top of the filling with the small pieces of butter. Roll out the second disc of dough for the top crust. You can make a lattice top, or place it whole over the filling. If using a whole top crust, cut several slits for steam to escape.

Trim the top crust, leaving overhang. Crimp the edges of both crusts together to seal. If desired, whisk the egg and water for an egg wash and brush over the top crust. Sprinkle with Turbinado sugar.

**Bake:** Place the pie on a baking sheet to catch any drips. Bake for 15-20 minutes at 400°F (200°C), then reduce the oven temperature to 375°F (190°C) and continue baking for another 40-50 minutes, or until the crust is golden brown and the filling is bubbly and tender. If the crust starts to brown too quickly, loosely cover the edges with foil.

Remove from oven and let cool completely on a wire rack for at least 3-4 hours before slicing and serving. This allows the filling to set properly.