Step 1: Prepare your pie crust dough. Divide into two equal portions, wrap in plastic, and refrigerate for at least 30 minutes.
Step 2: In a large bowl, combine sliced strawberries and chopped rhubarb. Add sugar, cornstarch, and salt. Toss gently to coat the fruit evenly. Set aside to macerate for 15-20 minutes.
Step 3: Preheat your oven to 400°F (200°C). On a lightly floured surface, roll out one portion of chilled pie dough into a 12-inch circle. Carefully transfer it to a 9-inch pie plate. Trim the edges, leaving a ½-inch overhang.
Step 4: Pour the fruit filling into the prepared pie crust, spreading it evenly. Dot the top of the filling with the small pieces of butter.
Step 5: Roll out the second portion of pie dough. You can either place it whole over the filling (cutting slits for venting) or create a lattice top with ¾-inch wide strips.
Step 6: Trim the edges of the top crust (or lattice) to match the bottom crust’s overhang. Fold the overhang under and crimp the edges decoratively to seal the pie.
Step 7: In a small bowl, whisk together the beaten egg and water (or milk) to create an egg wash. Brush the top crust generously with the egg wash, then sprinkle with 1-2 tablespoons of granulated sugar.
Step 8: Place the pie on a baking sheet to catch any drips. Bake for 15 minutes at 400°F (200°C).
Step 9: Reduce the oven temperature to 375°F (190°C) and continue baking for another 40-50 minutes, or until the crust is deeply golden brown and the filling is bubbly and thick. If browning too quickly, tent loosely with foil.
Step 10: Remove the pie from the oven and transfer it to a wire rack. Let it cool completely for at least 3-4 hours before slicing and serving. This is crucial for the filling to set properly.