Step 1: Prepare your pie crust dough. On a lightly floured surface, roll out one of the chilled pie dough discs (the larger one, if you have one) into a 12-inch circle. Carefully transfer it to a 9-inch pie plate. Trim the edges, leaving about a 1/2-inch overhang.
Step 2: Blind bake the bottom crust. Line the pie crust with parchment paper or foil, fill with pie weights, dried beans, or rice, and bake in a preheated oven at 375°F (190°C) for 15 minutes. Remove the weights and parchment/foil, then bake for another 5-10 minutes until lightly golden. Let cool completely.
Step 3: In a large bowl, combine the chopped rhubarb, granulated sugar, cornstarch, and salt. Gently toss until the rhubarb pieces are evenly coated. Set aside.
Step 4: Pour the rhubarb filling into the blind-baked pie crust, spreading it evenly. Mound it slightly in the center, as the rhubarb will cook down.
Step 5: Roll out the second dough disc for the top crust. You can either place it whole over the filling and cut several slits in the top for venting, or create a lattice top for a decorative finish. If making a lattice, cut the dough into 1-inch strips and weave them over the filling.
Step 6: Trim any excess dough from the top crust, then crimp the edges of the top and bottom crusts together to seal them. You can use your fingers or a fork.
Step 7: For a golden, shiny crust, brush the top with the beaten egg wash (if using) and sprinkle with coarse sugar (if using). This adds a beautiful finish and slight crunch.
Step 8: Place the pie on a baking sheet (to catch any potential drips) and bake in a preheated oven at 400°F (200°C) for 15 minutes. Then, reduce the oven temperature to 375°F (190°C) and continue baking for another 35-45 minutes, or until the crust is deeply golden brown and the filling is visibly bubbling.
Step 9: If the crust edges start to brown too quickly, loosely tent the pie with aluminum foil.
Step 10: Once baked, carefully transfer the rhubarb pie to a wire rack and let it cool completely for at least 3-4 hours before slicing and serving. This crucial step allows the filling to set properly, ensuring perfect slices.