In a food processor, pulse the flours, sugar, shortening, butter and salt until the butter and shortening are the size of small peas.
Whisk the egg, vinegar and water together in a large glass measuring cup.
Pour your wet ingredients into the food processor and pulse until the dough just comes together.
Dump the ball of dough onto a piece of parchment or plastic wrap. Divide the dough into two equal disks, wrap and chill the dough disks for at least 30 minutes.
After 30 minutes, roll out 1 disk of dough large enough to fit into your 9” pie dish.
Put dish in refrigerator to chill for 15 minutes.
Preheat oven to 425 degrees F.
Filling Preparation
Mix the rhubarb, strawberries, sugar, tapioca, flour, zest and juice of lemon, dash of cinnamon, and vanilla in a large bowl.
Pour into your chilled crust.
Dot the top of the filling with the butter.
Make an egg wash by whisking together the egg white and 1 tsp. water.
Roll out the other disk of dough.
Brush edges of pie crust with egg white wash.
Place the second dough over the filling and crimp the edges to seal. You can also make a lattice top for this pie, if you'd like!
Brush with egg white wash and garnish with coarse decorating sugar.
Protect the edge of the crust with foil and bake at 425 degrees F for 15 minutes.
Decrease temperature to 375 degrees F and bake for an additional 45 to 50 minutes, or until the filling starts bubbling. Higher altitude will take 450 degrees F and 400 degrees F respectively.