In a large bowl, stir together the flour, sugar, and salt until well blended. Cut in the shortening and butter until it resembles coarse crumbs, about the size of small peas.

Sprinkle 2 tablespoons of ice water over the mixture. Using a fork, gently stir in the water. Gradually add more water, 2 tablespoons at a time, just until the dough starts to come together.

Divide the dough in two portions, flatten each into a disc, wrap tightly in plastic wrap, and refrigerate for 1 hour.

At the end of the chilling time, preheat your oven to 425°F.

On a floured surface, roll out the bottom crust to fit a 9-inch pie plate, then transfer it into the pie plate.

In a mixing bowl, toss sliced rhubarb with sugar and flour until well coated. Pour this mixture into the prepared bottom crust and dot with pieces of butter.

Roll out the remaining crust to cover the filling, trim excess dough, and cut slits in the top crust for steam.

Beat the egg and brush it over the top crust.

Bake in the preheated oven for 15 minutes, then reduce temperature to 375°F and continue baking for 30-40 minutes.

Allow the pie to cool for at least 3 hours before serving.