In a large mixing bowl, combine the all-purpose flour and salt, then cut in the cold unsalted butter and cold vegetable shortening until the mixture resembles small peas.

Gradually add in ice cold water, just enough for the dough to come together without being sticky. Shape it into a disc, wrap in plastic wrap, and refrigerate for at least 1 hour.

Once chilled, remove the dough from the refrigerator and divide it into two equal discs. Wrap each disc in plastic wrap and refrigerate again for an additional 30 minutes.

Preheat your oven to 425°F (220°C) and adjust the oven rack to sit just above the center.

On a lightly floured surface, roll out one disc of the dough into a circle about 12 inches in diameter, then transfer it to a 9-inch pie plate.

In a large mixing bowl, combine the rhubarb, strawberries, light brown sugar, granulated sugar, cornstarch, vanilla extract, lemon juice, and unsalted butter. Stir gently until the fruit is coated.

Transfer the filling mixture into the chilled pie crust, spreading it evenly.

Roll out the second dough disc and drape it over the filling, trimming any excess dough and sealing the edges.

Brush the top crust with egg white, place the pie on a baking sheet, and bake at 425°F for 15 minutes, then lower the temperature to 375°F (190°C) for an additional 50-60 minutes.

Once baked, let the pie cool on a wire rack for at least 2 hours before serving.