Mix together the sliced rhubarb, sliced strawberries, and beaten egg. Set aside.
Whisk together the granulated sugar, flour, and quick-cooking tapioca. Set aside.
Roll out the pie pastry. Place one on the pie plate and trim if needed. Spread 1/4 cup of the sugar mixture over the pie pastry in the pan.
Add the strawberry rhubarb mixture to the pie plate and spread out in an even layer.
Spread the remaining sugar mixture over the fruit and top with a pie pastry. Trim excess, crimp the edges, and cut slits in the center. Brush with beaten egg white and sprinkle with sugar.
Bake at 425˚F for 15 minutes. Then at 350˚F for 40-50 minutes.