In a large bowl, combine the rhubarb, strawberries, sugar, tapioca flour, orange zest, and salt. Toss everything well and let it sit for 15–30 minutes so the tapioca can start working. After resting, pour off any excess juice that’s collected in the bottom of the bowl, this helps prevent a soggy pie.
If using homemade pie dough, let it rest at room temp for 10 minutes so it’s easier to roll out. Roll each disc to about ⅛ inch thick. Line a 9-inch pie plate with the bottom crust and trim any overhang.
Spoon the drained fruit filling into the prepared crust. Roll out the top crust and place it over the filling. Trim, crimp the edges to seal, and cut a few slits for steam. Or go full Martha and do a lattice. Brush the top with the beaten egg.
Preheat your oven to 425°F (218°C). Place the pie on the bottom rack and bake for 15 minutes. Reduce the temperature to 350°F (177°C) and continue baking for another 45–60 minutes, or until the crust is deep golden and the filling is bubbling through the vents.
Let the pie cool completely before slicing, at least a couple of hours. This gives the filling time to set up properly. It’ll still be juicy, just not a fruit landslide.
