Step 1: In a large bowl, whisk together the flour and salt.
Step 2: Add the cold butter cubes to the flour mixture. Using a pastry blender, your fingertips, or a food processor, cut the butter into the flour until the mixture resembles coarse meal with some pea-sized pieces of butter remaining.
Step 3: Gradually add the ice water, 1 tablespoon at a time, mixing until the dough just comes together. Do not overmix. The dough should still be a bit shaggy.
Step 4: Divide the dough in half, flatten each half into a disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes.
Step 5: Preheat your oven to 400°F (200°C) with a rack in the lower-middle position.
Step 6: In a large bowl, gently combine the sliced rhubarb and strawberries.
Step 7: In a separate small bowl, whisk together the granulated sugar, cornstarch (or tapioca starch), and cinnamon (if using).
Step 8: Pour the sugar mixture over the fruit and toss gently to coat evenly. Stir in the vanilla extract. Let the fruit sit for 10-15 minutes.
Step 9: 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 a ½-inch overhang.
Step 10: Pour the fruit filling into the pie crust. Dot the top of the filling with the small pieces of butter.
Step 11: Roll out the second disc of pie dough. Place it whole over the filling (cut a few slits for steam vents) or create a lattice top. Trim any excess dough, then crimp the edges of the top and bottom crusts together to seal.
Step 12: Brush the top crust with the egg wash and sprinkle generously with coarse sugar.
Step 13: Place the pie on a baking sheet. Bake for 20 minutes at 400°F (200°C).
Step 14: Reduce the oven temperature to 375°F (190°C) and continue baking for another 40-50 minutes, or until the crust is deeply golden brown and the filling is bubbling thickly. If needed, loosely cover the edges with aluminum foil.
Step 15: Remove the pie from the oven and let it cool completely on a wire rack for at least 3-4 hours before slicing and serving. This allows the filling to set properly.