Step 1: Prepare the Pie Crust. In a large bowl, whisk together 2 1/2 cups flour and 1 teaspoon salt. Add the cold, cubed butter. Using a pastry blender or your fingertips, cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 2: Gradually add the ice water, 1 tablespoon at a time, mixing lightly with a fork or your hands until the dough just comes together. Be careful not to overmix. The dough should be shaggy but hold together when squeezed.
Step 3: Divide the dough in half, form each half into a flattened disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes, or up to 2 days.
Step 4: Prepare the Rhubarb Filling. In a large bowl, combine the chopped rhubarb, granulated sugar, 1/4 cup flour, cornstarch, cinnamon, nutmeg, and a pinch of salt. Toss gently to ensure all rhubarb pieces are evenly coated. Set aside.
Step 5: Assemble the Pie. On a lightly floured surface, roll out one disk of chilled pie dough into a 12-inch circle. Carefully transfer the dough to a 9-inch pie plate. Trim the edges, leaving about a 1/2-inch overhang.
Step 6: Pour the rhubarb filling into the pie crust, mounding it slightly in the center. Ensure the filling is evenly distributed.
Step 7: Roll out the second disk of pie dough. You can place it directly over the filling and cut several slits for venting, or create a lattice top for a decorative finish.
Step 8: Place the top crust over the filling. Trim the edges to match the bottom crust, then crimp them together to seal using your fingers or a fork. For a golden, shiny crust, brush the top with the egg wash and sprinkle with 1 tablespoon of granulated sugar (optional).
Step 9: Preheat your oven to 400°F (200°C). Place the pie on a baking sheet to catch any potential drips.
Step 10: Bake the pie for 15 minutes at 400°F (200°C). Then, reduce the oven temperature to 375°F (190°C) and continue baking for another 45-55 minutes, or until the crust is golden brown and the filling is bubbling thickly. If the crust edges start to brown too quickly, loosely cover them with aluminum foil.
Step 11: Once baked, remove the rhubarb pie from the oven and place it on a wire rack to cool completely for at least 3-4 hours. This crucial step allows the filling to set properly, ensuring clean slices. Serve warm or at room temperature.