Prepare the Pie Dough: Whisk the flour and salt in a large bowl. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

Slowly add the ice water, 1 tablespoon at a time, mixing gently until the dough just comes together. Divide the dough in half, form each half into a disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes.

Prepare the Filling: In a large bowl, combine the chopped rhubarb, sliced apples, granulated sugar, cornstarch, lemon zest, lemon juice, and cinnamon. Toss gently until the fruit is evenly coated. Set aside.

Assemble the Pie: Preheat your oven to 400°F (200°C). On a lightly floured surface, roll out one disk of dough to an 12-inch circle. Carefully transfer it to a standard 9-inch pie dish. Trim the edges, leaving about a 1-inch overhang.

Pour the fruit filling into the bottom crust. Roll out the second disk of dough and carefully place it over the filling. Crimp the edges of the top and bottom crusts together to seal well. Cut several slits in the top crust to allow steam to escape.

Bake the Pie: Brush the top crust with the beaten egg wash and sprinkle with coarse sugar, if using. Place the pie on a baking sheet (to catch drips). Bake for 20 minutes at 400°F (200°C). Then, reduce the oven temperature to 375°F (190°C) and bake for an additional 40-45 minutes, or until the crust is deep golden brown and the filling is bubbling thickly.

Cooling: Let the pie cool on a wire rack for at least 3 hours before slicing. This allows the filling to set properly. Serve warm or at room temperature.