Remove pie shell from freezer and allow to thaw at room temperature for 10 - 15 minutes.
Remove pie shell from foil plate and place on large sheet of parchment paper.
Roll out pie shell to approximately 10-10-1/2 inches (25-26 cm) in diameter. Edge can be uneven. Slide pie shell and paper onto a large flat baking sheet; pastry may hang over edge of sheet.
Blot strawberry and rhubarb slices well with paper towel to remove any excess moisture before combining with zest and a mixture of flour, sugar and salt.
Spoon fruit mixture into centre of pastry, leaving a pastry edge that is 3-inches (8 cm) wide. Fold pastry edge up and over fruit mixture, overlapping edges as required, leaving centre of pie uncovered. Trim excess parchment paper around edge of pie to prevent burning.
Brush pastry with beaten egg and sprinkle pastry and fruit filling with coarse sugar, if using.
Bake on lowest shelf of preheated oven for 30 minutes at 375 degrees F (190 degrees C) or until fruit is bubbling and pastry is golden brown. Allow to stand for 5 minutes before serving. Pie is best served warm the day it is made.
