In a deep bowl, combine the flour and salt. Work the butter and shortening into the flour mixture with your hands (yes, your bare hands) until marble-size lumps form.
Add the water a little at a time, using only enough to moisten the dough by sort of “fluffing” the flour to evenly distribute and work in the water. The mixture should hold together when squeezed and have a tacky consistency. (Do not overwork the dough. It takes very little time to come together and you’ll be tempted to keep touching it, but don’t.)
Divide the dough in 2 and form each piece into a disk shape (think hamburger patty). The dough should be slightly tacky. If it is too sticky, rub your hands with flour.
On a floured surface and with a floured rolling pin, roll the dough into an 11-inch circle. When rolling, angle the rolling pin to coax the edges to maintain a circular shape. Sprinkle flour under and on top of the dough to keep it from sticking to the surface.
Loosen the dough from the surface and carefully transfer it to a 9- or 10-inch pie plate. Fit the dough into the plate (avoid stretching). Using scissors, trim the dough to a 1-inch overhang.
Roll out the second disk of dough.
Heat oven to 425° F. In a large bowl, toss together the rhubarb, sugar, cornstarch, and salt.
Fill the prepared bottom crust with the fruit and top with the butter. Lay the second crust on top, gently pressing the dough to fit snuggly over the fruit. Lift the edges of both the top and bottom crusts and roll them together, underhanded, all the way around the dish. Crimp the edges by pinching the dough with the side of your index finger against the thumb and forefinger to create a fluted pattern. Alternatively, press down the edge of the dough with a fork. (The point is to create a seal so the juices don’t run out while baking.) Brush the crust with some of the egg (but be careful not to let the egg pool in the crevices of the edge). Cut vents in the top.
Place the pie on a foil-lined rimmed baking sheet and bake for 20 minutes to “set” and brown the crust. Reduce oven to 375° F and bake until the fruit mixture is bubbling and the crust is golden brown, 55 to 60 minutes. Let cool. Then eat!