In a large bowl, whisk together flour, salt, and sugar. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs with some pea-sized pieces remaining.

Gradually add ice water, one tablespoon at a time, mixing until the dough just begins to come together. Divide the dough into two equal discs, wrap in plastic, and refrigerate for at least 1 hour.

Preheat your oven to 400°F (200°C). On a lightly floured surface, roll out one disc of dough to a 12-inch circle and fit it into a 9-inch pie plate.

In a large mixing bowl, toss together the strawberries, rhubarb, sugar, cornstarch, vanilla, cinnamon, and lemon juice until the fruit is evenly coated.

Pour the fruit mixture into the prepared bottom crust. Roll out the second disc of dough and place it over the filling. Trim, fold, and crimp the edges to seal the pie.

Cut several slits in the top crust to allow steam to escape. Brush the top with the beaten egg wash and sprinkle with turbinado sugar.

Bake at 400°F for 20 minutes, then reduce the oven temperature to 375°F (190°C) and bake for an additional 30-35 minutes until the crust is golden brown and the filling is bubbling through the vents.

Remove from the oven and let the pie cool completely on a wire rack for at least 4 hours. This is crucial for the filling to set properly before slicing.