Step 1: Prepare your pie crust. Roll out one disc of dough and carefully fit it into a 9-inch pie plate. Trim the edges, leaving about a 1/2-inch overhang. Place the pie plate with the bottom crust in the refrigerator while you prepare the filling.
Step 2: Prepare the rhubarb. Wash the rhubarb stalks thoroughly and pat them dry. Trim off any tough ends and the leaves (which are toxic). Slice the rhubarb into 1/2 to 3/4-inch pieces. You should have approximately 6 cups of chopped rhubarb. Place the chopped rhubarb in a large bowl.
Step 3: Make the filling. In a separate medium bowl, whisk together the granulated sugar, all-purpose flour (or cornstarch), ground cinnamon, and a pinch of salt until well combined.
Step 4: Combine wet and dry ingredients. Pour the sugar-flour mixture over the chopped rhubarb. Toss gently with a spoon or your hands until all the rhubarb pieces are evenly coated. Let it sit for about 10-15 minutes.
Step 5: Assemble the pie. Retrieve your chilled pie crust. Pour the rhubarb filling evenly into the pie crust. Dot the top of the filling with small cubes of unsalted butter.
Step 6: Add the top crust. Roll out the second disc of pie dough. You can either place it directly over the filling, cut vents for steam, or create a lattice top. Carefully place the top crust over the filling.
Step 7: Seal and crimp. Trim the edges to match the bottom crust, leaving a 1/2-inch overhang. Fold the overhang of both crusts under, flush with the rim of the pie plate. Crimp the edges decoratively using your fingers or a fork to seal the pie. Cut several slits in the top crust (if not using a lattice) to allow steam to escape.
Step 8: Egg wash and sugar sprinkle (optional). For a beautiful golden-brown and sparkling crust, whisk one egg with a tablespoon of water to create an egg wash. Brush this lightly over the top crust, then sprinkle generously with granulated sugar.
Step 9: Bake the rhubarb pie. Preheat your oven to 400°F (200°C). Place the pie on a baking sheet. Bake for 15-20 minutes at 400°F, then reduce the oven temperature to 375°F (190°C) and continue baking for another 40-50 minutes, or until the crust is golden brown and the filling is bubbly. If the crust starts to brown too quickly, loosely cover the edges with aluminum foil.
Step 10: Cool the pie. Transfer the pie to a wire rack and let it cool completely for at least 3-4 hours before slicing. This allows the filling to set properly.