Make the Pie Crust: In a large bowl, whisk together the flour and salt. Add the cold, cubed butter and cut it into the flour using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Gradually add the ice water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix.

Chill the Dough: Divide the dough in half, flatten each half into a disc, wrap in plastic wrap, and refrigerate for at least 2 hours or overnight.

Prepare the Filling: In a large bowl, combine the rhubarb, strawberries, sugar, flour, and salt. Gently toss to coat the fruit evenly. Stir in the lemon juice.

Roll Out the Bottom Crust: On a lightly floured surface, roll out one of the dough discs into a 12-inch circle. Carefully transfer the dough to a 9-inch pie plate. Trim the edges, leaving about a 1-inch overhang.

Add the Filling: Pour the strawberry rhubarb mixture into the pie crust. Dot the top with the small pieces of butter.

Roll Out the Top Crust: Roll out the second dough disc into a 12-inch circle. Cut several slits in the dough to allow steam to escape during baking.

Cover the Pie: Carefully place the top crust over the filling. Trim the edges, leaving about a 1-inch overhang.

Crimp the Edges: Crimp the edges of the crust together to seal. You can use a fork to press around the edges or create a decorative crimp with your fingers.

Egg Wash and Sprinkle: Brush the top crust with the beaten egg (egg wash) and sprinkle with coarse sugar.

Bake the Pie: Bake in a preheated oven at 400°F (200°C) for 15 minutes. Then, reduce the oven temperature to 375°F (190°C) and continue baking for another 45-50 minutes, or until the crust is golden brown and the filling is bubbling.

Cool Completely: Let the Strawberry Rhubarb Pie cool completely on a wire rack before slicing and serving.