Line a baking tray with parchment paper and set aside.
In a large bowl, whisk together sugar and flour. Add rhubarb and strawberries, toss to combine. Allow to rest at room temperature for 30 minutes, stirring after 15 minutes.
Meanwhile, preheat oven to 400˚F.
If using a refrigerated crust, add chilled bottom crust to pie tin and refrigerate until ready to fill. Remove pie shell from freezer if frozen.
Add rhubarb and strawberry mixture into the pie shell. Place small dabs of butter all over the fruit mixture. Carefully place the other half of prepared dough on the top of the fruit pie. Press and crimp edges to seal. Cut three small slits on top to allow steam to escape.
Lightly beat the egg and milk together. Brush top of the crust with egg wash and sprinkle with sanding sugar if using.
Place pie on parchment lined baking tray. Bake for 30-45 minutes, or until golden brown and fruit is cooked through. Allow to cool completely before serving.
Serve with ice cream or whipped cream.
Notes
Leftover pie can be kept tightly covered at room temperature for up to 2 days or refrigerated for up to 3 days.
Freeze individual portions in freezer-safe containers and thaw them in the refrigerator.