Preheat the oven. Place rack in the middle and preheat the oven to 400°F.

In a large bowl, gently combine the rhubarb and the strawberries with the sugar, tapioca, salt, and orange zest. Let sit for 10 minutes.

Roll out one disc of pastry dough and line the bottom of a pie dish with it. Trim to ½ inch from the edge. Transfer the filling into the crust and spread out evenly.

Roll out the second disc of pastry dough and place over the pie. Trim the edges to an inch from the edge of the pie dish. Tuck the top crust edges over the bottom crust edges and use your fingers or a fork to crimp the top and bottom edges together. Cut slits in the top for the steam to escape. Brush a thin layer of egg white or cream over the top of the pie if desired.

Place pie on the middle rack of the 400°F oven with a baking sheet on a lower rack to catch any juices that might spill over. Bake for 20 minutes at 400°F, then reduce heat to 350°F and bake an additional 40-50 minutes longer (50 to 60 minutes longer if doing a 10-inch pie).

Remove from oven and let cool on a rack. Serve warm or cold. Pie can also be covered and stored at room temperature for up to 2 days or refrigerated for 5 days.