In a large mixing bowl, combine the gluten-free flour, sugar, and salt.
Add the cold butter cubes and use a pastry cutter or your fingers to work the butter into the flour until the mixture resembles coarse crumbs.
Gradually add the cold water, mixing until the dough comes together.
Divide the dough into two balls, wrap in plastic wrap, and refrigerate for at least 30 minutes.
In another bowl, combine the strawberries, rhubarb, sugar, cornstarch, lemon juice, and lemon zest. Mix well and let sit for about 15 minutes.
Preheat your oven to 375°F (190°C).
Roll out one of the dough balls on a floured surface and fit it into a pie dish.
Pour the filling into the crust.
Roll out the second dough ball and place it over the filling. Trim any excess dough and crimp the edges to seal.
Cut a few slits in the top crust to allow steam to escape.
Brush the top crust with beaten egg and sprinkle with sugar, if desired.
Place the pie on a baking sheet to catch any drips and bake in the preheated oven for 45-50 minutes, or until the crust is golden and the filling is bubbling.
Let the pie cool completely before slicing and serving.