01 - In a large mixing bowl, whisk together the gluten-free flour blend, sugar, and salt. Add the cold cubed butter and cut it in using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
02 - Beat the egg with 4 tablespoons of ice water in a small bowl. Gradually drizzle the egg mixture into the flour, stirring gently until the dough just begins to come together. Add remaining ice water one tablespoon at a time if the dough feels dry. Divide into two equal discs, wrap tightly in plastic wrap, and refrigerate for at least 1 hour.
03 - Preheat the oven to 400°F. Position a rack in the lower third of the oven for optimal bottom crust browning.
04 - In a large bowl, combine the sliced strawberries, diced rhubarb, granulated sugar, cornstarch, cinnamon, salt, and lemon juice. Toss gently until the fruit is evenly coated and the cornstarch is fully distributed. Set aside.
05 - Roll out one disc of chilled dough between two sheets of parchment paper to a 12-inch circle, approximately 1/8 inch thick. Carefully transfer the dough to a 9-inch pie plate, gently easing it into the bottom and sides. Trim any excess dough leaving a 1-inch overhang.
06 - Pour the strawberry-rhubarb filling into the lined pie plate, spreading it into an even layer. Scatter any remaining juices from the bowl over the fruit.
07 - Roll out the second disc of dough and drape it over the filling for a solid top, or cut into strips and weave a lattice. Fold the bottom crust overhang over the top edge and crimp to seal. Cut several steam vents in the top crust if using a solid lid. Brush the surface evenly with the beaten egg and sprinkle with coarse sugar.
08 - 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 deeply golden and the filling is bubbling through the vents. Cover the edges with aluminum foil if they brown too quickly.
09 - Transfer the pie to a wire cooling rack and allow it to rest for at least 2 hours before slicing. This resting time allows the filling to set properly. Serve warm or at room temperature.