In a large bowl, whisk together the flour and salt. Add the cold butter cubes and cut them into the flour mixture 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, 1 tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix. The dough should be shaggy but hold together when squeezed.

Divide the dough in half, flatten each half into a disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes, or up to 2 days.

In a large bowl, combine the sliced peaches and quartered strawberries. Add the granulated sugar, brown sugar, cornstarch, lemon juice, vanilla extract, cinnamon (if using), and a pinch of salt. Gently toss until the fruit is evenly coated.

Let the fruit mixture sit at room temperature for 15-20 minutes, allowing the sugars to draw out juices from the fruit. This will create a delicious syrup.

Preheat your oven to 425°F (220°C). Place a baking sheet on the bottom rack of the oven to catch any drips.

On a lightly floured surface, roll out one disk of chilled pie dough into a 12-inch circle. Carefully transfer it to a 9-inch pie dish. Trim the edges, leaving about a 1/2-inch overhang.

Pour the strawberry peach filling into the pie crust, arranging the fruit evenly. Dot the top of the filling with the small pieces of unsalted butter.

Roll out the second disk of pie dough. You can either place it whole over the filling and cut several slits for steam vents, or cut it into strips to create a lattice top. Carefully place the top crust over the filling.

Trim the edges of the top crust to match the bottom, then fold the overhang under and crimp the edges together to seal the pie. For a decorative edge, you can use a fork or your fingers.

In a small bowl, whisk together the beaten egg and milk/water to create an egg wash. Brush the top crust evenly with the egg wash, then sprinkle with 1 tablespoon of granulated sugar for a sparkling finish.

Place the assembled pie on the preheated baking sheet in the oven. Bake at 425°F (220°C) for 15 minutes.

Reduce the oven temperature to 375°F (190°C) and continue to bake for another 40-50 minutes, or until the crust is deeply golden brown and the filling is bubbly and thickened. If the crust starts to brown too quickly, loosely tent the pie with aluminum foil.

Once baked, carefully remove the pie from the oven and place it on a wire rack to cool completely for at least 3-4 hours before slicing and serving. This allows the filling to set properly.