Mix the flour, sugar and salt very well in a large mixing bowl. Cut the butter into small bits and scatter over the flour mixture. Alternatively, use a grater and grate the cold butter in, tossing occasionally to mix. Stir until coarse.
Add about 3 tablespoons of water and mix with a spoon or heavy-duty scraper. Keep adding water until flour mixture holds together when you pinch it with your fingers.
Dump the mixture onto a pastry mat and press it all together. It shouldn't crumble at this point.
Wrap in plastic wrap and let it rest in the refrigerator for at least 30 minutes. For best results, let it chill for a few hours.
About an hour or so before you plan to assemble, bring dough to room temperature.
Prepare the rhubarb and strawberry filling. Quick thaw your frozen rhubarb and strawberries by putting them in a mixing bowl and pouring 2 cups of boiling water over the fruit. Let stand for 5 minutes, then drain. Toss the now-mostly-thawed rhubarb and strawberries with sugar and orange zest and allow mixture to stand for about 30 minutes. Drain and measure so that you have between 2/3 and 1 cup of liquid. If you don’t have at least 2/3 cup let stand for another 15 minutes or so.
Preheat oven to 425 degrees F.
Pour the reserved 2/3 to 1 cup rhubarb/strawberry juice mixture into a saucepan and whisk or stir in 1 tablespoon of cornstarch. Turn heat to medium and begin whisking or stirring until mixture becomes thick and translucent. Remove from heat. Let cool slightly while you prepare the dough.
On a well-floured surface, roll dough into a large (about 12 inches) circle.
Line a large rimmed baking sheet (make sure it’s rimmed to catch any leaks) with nonstick foil or parchment. Transfer your dough circle to the baking sheet. The best way to do this is by using a bench scraping, folding it in quarters, then unfolding onto the foil lined baking sheet.
Mix the thickened fruit juice mixture with strawberries and rhubarb, then spoon the fruit mixture into the center of the dough leaving about a 2 1/2 inch border. At this point you may dot with bits of butter if using. Fold border up and around the fruit, leaving the middle part open to the fruit filling is exposed.
Brush the crust with lightly beaten egg and sprinkle with coarse sugar.
Bake for about 25 minutes or until golden brown. Let cool to room temperature before serving.