I do like to use quick-cooking tapioca for many juicier fruit pies, but if you have trouble finding it, feel free to use cornstarch instead. And let the bubbling juices be your guide as to when the pie is truly done; some pies may take just a few minutes longer than others.
Prepare your pie: Roll out 2 dough disks to 12-inch round; transfer one right into a 9-inch pie pan and trim any excess overhang (leave 1/2-inch overhang), leave the second one out as it will top the pie once filled.
Preheat oven to 400°F. Combine first seven ingredients (along with the optional orange zest) in a large bowl and gently toss to combine. Spoon filling into bottom crust. Scatter little pats of butter on top of the filling and place the second disk of dough on top of the pie. Fold edges under and crimp as desired. Brush with egg mixture and cut a few slits in the top of the pie to allow steam to escape.
Bake 20 minutes. Reduce oven temperature to 350 F and bake pie until the top is golden and the fruit filling starts to bubble, about another 25 minutes. Transfer pie to rack and cool completely.