01 - In a large mixing bowl, whisk together the gluten-free flour blend, sugar, and salt until evenly combined.
02 - Add the cubed cold butter to the flour mixture. Using a pastry blender or your fingertips, cut the butter into the dry ingredients until the texture resembles coarse meal 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-butter mixture, tossing gently with a fork just until the dough begins to come together. Add remaining ice water 1 tablespoon at a time if the dough feels dry. Divide the dough into two equal portions, shape each into a flat disc, wrap tightly in plastic wrap, 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 even bottom browning.
05 - In a large bowl, combine the sliced strawberries, diced rhubarb, sugar, cornstarch, cinnamon, salt, and lemon juice. Toss gently until the fruit is evenly coated and the cornstarch is fully distributed. Set aside.
06 - Place one disc of chilled dough between two sheets of parchment paper and roll out to a 12-inch circle, about ⅛ inch thick. Carefully transfer the dough to a 9-inch pie plate, gently easing it into the corners. Trim any excess dough hanging over the edge, leaving about ½ inch for sealing.
07 - Pour the strawberry rhubarb filling into the bottom crust, spreading it into an even layer with any accumulated juices.
08 - Roll out the second disc of dough and drape it over the filling. Create a lattice pattern or leave solid, cutting a few small steam vents in the center. Fold the top edge under the bottom crust edge and crimp firmly to seal. Brush the entire top crust with the beaten egg and sprinkle evenly with coarse sugar.
09 - Place the assembled 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 brown and the filling is bubbling through the vents. If the edges brown too quickly, cover them with a foil ring after the first 25 minutes.
10 - Transfer the pie to a wire cooling rack and allow it to cool for at least 2 hours before slicing. This resting time allows the filling to set properly. Serve warm or at room temperature.