Line a heavy-duty rimmed baking sheet or pizza stone with foil. Place in the oven on the middle rack. Preheat oven to 400°F.
In a large mixing bowl combine the rhubarb and granulated sugar. Microwave the rhubarb on high for 1 minute. Remove and stir then microwave on high, one minute more. Add the strawberries and fold gently to combine. Set aside for 30 minutes, stirring halfway through.
While the rhubarb and strawberries macerate, roll out half of the prepared pie dough. Roll the pastry into a circle large enough to fit in a deep dish pie plate with a 1-inch overhang. Gently ease the pastry into the pie plate taking care not to stretch the dough while pressing into the corners. Place the pie plate and pastry in the refrigerator to chill until needed.
Add the tapioca, salt and orange zest to the rhubarb and strawberry mixture. Fold gently to combine. Pour the fruit and any juices into the prepare pie crust. Return the pie plate to the refrigerator to keep cool while rolling out the top crust.
Roll the top crust into a circle about 2-inches larger than the top of the pie plate. Cut into strips for a lattice crust and create a weave design over the fruit. Trim the excess dough and fold the over-hang up over the lattice pieces. Pinch together to seal and crimp into a decorative edge, as desired.
Brush the pie crust lightly with 1 egg beaten with 1 teaspoon of milk. Sprinkle with coarse sugar and place on the prepared baking sheet in the oven.
Bake at 400°F for 25 minutes. Reduce the temperature to 350°F and continue baking for an additional 40 to 45 minutes or until the crust is golden brown and the juices are bubbling.
Remove from the oven and allow the pie to cool on a rack at room temperature. Serve slightly warm, room temperature or chilled. Top with a scoop of ice cream, if desired.