In a large mixing bowl, combine the sliced rhubarb and strawberries with light brown sugar, granulated sugar, cornstarch, vanilla extract, and lemon juice. Stir gently to coat the fruit evenly. Set aside.

In another bowl, whisk together all-purpose flour and salt. Cut in the cold unsalted butter and vegetable shortening until the mixture resembles coarse crumbs. Gradually add ice-cold water until the dough just comes together. Wrap in plastic and refrigerate for at least 1 hour.

Preheat your oven to 425°F (220°C).

On a floured surface, roll one disk of dough into a 12-inch circle and transfer it into a 9-inch pie plate. Trim any excess edges.

Pour the fruit mixture into the pie crust, dot the top with butter, and cover with the second disk of rolled dough, trimming any excess.

Seal the edges of the pie and brush the top crust with beaten egg white.

Bake at 425°F (220°C) for 15 minutes, then lower to 350°F (175°C) and bake for an additional 30-35 minutes.

Once baked, remove from the oven and let cool for 2 hours on a wire rack.