Stir together the flour, sugar, and salt in a large bowl. Using a pastry cutter or a food processor, cut the shortening and butter into the flour until the mixture resembles coarse crumbs. It is only about 10 pulses with a food processor but a bit more work by hand.

Sprinkle 2 tablespoons of the ice water over the mixture. Using a fork, gently stir the water in to the flour mixture, then repeat with the remaining water, adding only 2 tablespoons at a time, just until the dough starts to come together.

Divide the dough in two. Turn each piece of dough onto a sheet of plastic wrap and flatten each into a disc, then wrap it tightly and refrigerate for 1 hour until ready to use.

Roll out the bottom crust on a floured surface, using additional flour on top of the crust as needed to keep it from sticking to the rolling pin. It should be slightly larger than a 9-inch pie plate. Start by working from the center of the dough out toward the edges and just press the crust back together if you get tears. Transfer the bottom crust to the pie plate. This is easiest to do by first loosening the crust from the counter by sliding a bench scraper or thin metal spatula under the crust, then rolling the crust onto your rolling pin to lift and move it over to the pie plate. Set aside.

In a large bowl, toss the rhubarb with the sugar and flour, then pour all of it into the bottom pie crust. Cut the butter into small pieces and distribute them around the top of the filling.

Roll out the remaining crust, then transfer it to cover the top of the pie filling. Trim the edges of the crust, if necessary, and tuck them under for a clean finish. Crimp or press the edges of the crust in a decorative manner. Cut slits for vents in the top of the pie.

Beat the egg and brush it over the top of the pie. This will give the crust a nice golden finish. You can even sprinkle the pie with 1-2 tablespoons of granulated sugar or coarse sugar for a sparkly, crunchy finish on top.

Bake for 15 minutes, then reduce the oven temperature to 375 degrees F without opening the oven door. Continue to bake for 30-40 minutes until the crust is golden brown and the filling is bubbling.

Remove the pie from the oven and cool at least 3 hours (or even longer) for the pie to set up. Slice and serve.