In a large mixing bowl, whisk together all-purpose flour and salt. Cut in very cold unsalted butter and vegetable shortening until the mixture resembles small peas.

Gradually incorporate ice-cold water, mixing just until the dough comes together. Divide the dough into two discs, wrap in plastic, and refrigerate for at least 1 hour.

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

Roll out one disc of dough to fit a 9-inch pie plate and return it to the fridge to chill.

Combine sliced rhubarb and strawberries with light brown sugar, granulated sugar, cornstarch, vanilla extract, lemon juice, and unsalted butter. Toss gently.

Transfer the fruit filling into the chilled pie crust and cover with the second disc of dough, crimping the edges to seal.

Brush the top crust with whisked egg white and cut small slits for venting.

Bake at 425°F for 15 minutes, then reduce to 375°F and continue baking for 50-60 minutes.

After baking, let the pie cool on a wire rack for at least 2 hours.