Instructions:
- Prepare the Pastry Dough: In a food processor or large bowl, pulse/cut the cold butter into the flour, sugar, and salt until the mixture resembles coarse crumbs with pea-sized butter pieces remaining. Gradually add the ice water/vinegar mixture until the dough just comes together. Divide dough in half, form into flat discs, wrap tightly, and chill for at least 2 hours.
- Prepare the Fruit Filling: Gently combine strawberries and rhubarb in a large bowl. In a separate small bowl, whisk together both sugars, cornstarch, salt, lemon zest, and black pepper. Pour the dry mixture over the fruit. Add vanilla extract. Toss gently until evenly coated. Let the mixture sit for 20 minutes.
- Assemble the Bottom Crust: Remove one dough disc. On a lightly floured surface, roll out to a 12-inch circle. Carefully transfer the dough to a 9-inch pie plate. Trim the overhang to about 1 inch past the rim. Chill again while preparing the top crust.
- Fill and Top the Pie: Preheat oven to 400°F (200°C). Place a baking sheet on the bottom rack. Pour the fruit filling (including accumulated juices) into the chilled bottom crust. Roll out the second dough disc and place it over the filling. Crimp the edges decoratively to seal. Cut vents in the top crust.
- Finish and Bake: Brush the entire top crust liberally with the egg wash. Sprinkle generously with coarse turbinado sugar. Start baking at 400°F (200°C) for 20 minutes. Reduce the oven temperature to 375°F (190°C) and continue baking for another 40–55 minutes until the crust is deep golden brown and the filling is bubbling thickly.
- Cool Completely: Cool the pie on a wire rack for a minimum of 4 hours (preferably 6 hours) before slicing to allow the cornstarch to fully set the juices.