Make the pie dough.In large bowl, whisk together flour, salt and sugar. Add chilled, cubed butter and mix with finger tips or pastry blender until the consistency of coarse crumbs. Sprinkle with ice water, one tablespoon at a time until dough can come together in a ball. Divide in two, flatten discs and wrap in plastic wrap. Chill in refrigerator for 30 minutes.
Meanwhile, make the filling. Combine sliced strawberries and rhubarb in bowl. Add sugar, tapioca, vanilla and zest (if using) and toss gently and thoroughly to combine. Let sit about 20 minutes.
Preheat oven to 400ºRoll out one disc of dough on floured board. Place in bottom of 9-inch glass pie plate, dough will hang over edges of plate. Spoon in strawberry-rhubarb filling and all accumulated juices. Dot with one tablespoon of butter.
Roll out second disc of dough for top crust. Carefully lay over filling, seal edges by trimming and folding/pinching dough edges together. Crimp with fork tines or with fingers. With tip of sharp knife, cut 4 or 5 slits in top of crust and a tiny hole in the middle so pie filling can vent while baking.
Whisk together egg wash (egg and milk or water) and brush lightly all over top of pie. Sprinkle with sugar.
Bake in oven for 20 minutes, then rotate pie and turn oven down to 350º. Bake another 30 to 35 minutes until golden brown and filling is thick and bubbly through the vents.
Cool on rack. It will thicken more as it cools.