Preheat the oven to 450°F
Lightly flour the bottom of a 9 inch pie pan
Roll out one pie crust if needed
Place it into the pie pan and press into place
Trim the edges to about 1/2 inch beyond the rim of the pie pan
Refrigerate while preparing the filling
In a large bowl, combine rhubarb, flour, sugar, salt, and orange peel if using
Stir until evenly coated
Pour the filling into the chilled pie crust
Evenly distribute any remaining sugar mixture over the rhubarb
Dot the filling with butter
Roll out the second pie crust if needed
Place it over the filling
Use a little water around the edge to seal the crusts together
Trim excess dough to match the bottom crust
Crimp or pinch the edges to seal
Whisk the egg with a splash of water
Brush over the top crust
Cut several small slits in the top crust for venting
Bake for 15 minutes at 450°F
Reduce the oven temperature to 350°F
Bake for another 25 to 30 minutes until golden brown and bubbling
Cool before slicing and serving