01 - In a large mixing bowl, whisk together the gluten-free flour blend, sugar, and salt until evenly distributed.
02 - Add the cold cubed butter to the flour mixture. Using a pastry blender or your fingertips, cut the butter in until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
03 - In a small bowl, beat the egg with 4 tablespoons of ice water. Gradually drizzle the egg mixture into the flour mixture, stirring gently until the dough just begins to come together. Add remaining ice water 1 tablespoon at a time if the dough feels dry. Divide the dough into two equal discs, wrap tightly in plastic, and refrigerate for at least 1 hour.
04 - Preheat the oven to 400°F. Position a rack in the lower third of the oven for optimal bottom browning.
05 - In a large bowl, combine the sliced strawberries, diced rhubarb, granulated sugar, cornstarch, cinnamon, salt, and lemon juice. Toss gently until all the fruit is evenly coated and the cornstarch is fully incorporated.
06 - Roll out one disc of chilled dough between two sheets of parchment paper to a 12-inch round. Carefully ease the dough into a 9-inch pie plate, gently pressing it into the bottom and sides. Trim any excess dough leaving a 1-inch overhang.
07 - Pour the strawberry-rhubarb filling into the bottom crust, spreading it into an even layer and mounding it slightly in the center.
08 - Roll out the second disc of dough and drape it over the filling as either a solid top or a woven lattice. Fold the overhang under itself and crimp the edges decoratively to seal. Brush the top crust with beaten egg and sprinkle evenly with coarse sugar.
09 - Place the pie on a rimmed baking sheet to catch any drips. Bake at 400°F for 45 to 50 minutes until the crust is deep golden and the filling is bubbling through the vents. Tent loosely with foil if the crust browns too quickly.
10 - Transfer the pie to a wire cooling rack and let it rest for at least 2 hours before slicing to allow the filling to set. Serve warm or at room temperature.