Step 1: In a large bowl, whisk together the all-purpose flour, 2 tablespoons granulated sugar, and 1 teaspoon salt.
Step 2: Add the very cold, cubed unsalted butter to the flour mixture. Using a pastry blender, your fingertips, or a food processor, cut the butter into the flour until the mixture resembles coarse meal with some pea-sized pieces of butter remaining.
Step 3: Gradually add the ¾ cup ice water, one tablespoon at a time, mixing gently until the dough just comes together. Be careful not to overmix. Divide the dough into two equal portions, flatten each into a disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes, or up to 2 days.
Step 4: In a separate large bowl, gently combine the mixed berries, ¾ cup granulated sugar, ¼ cup cornstarch (or tapioca starch), and a pinch of salt. Gently toss until the berries are evenly coated. If desired, stir in the lemon zest.
Step 5: Preheat your oven to 400°F (200°C). Lightly grease a 9x13 inch baking sheet or line with parchment paper.
Step 6: On a lightly floured surface, roll out one disc of chilled pie dough into a rectangle roughly 10x14 inches. Carefully transfer the dough to the prepared baking sheet, gently pressing it into the bottom and slightly up the sides.
Step 7: Pour the berry filling evenly over the bottom crust. Dot the small pieces of butter (2 tablespoons total) over the berry filling.
Step 8: Roll out the second disc of dough to a similar size (10x14 inches). Place it directly over the berries for a solid top crust, or cut it into strips to create a lattice top. If making a lattice, weave the strips over the filling. Trim and crimp the edges of both crusts together to seal.
Step 9: In a small bowl, whisk the egg with 1 tablespoon of water to create an egg wash. Brush the top crust generously with the egg wash. Sprinkle the top with the coarse turbinado sugar.
Step 10: Cut a few small slits in the top crust (if not using a lattice) to allow steam to escape during baking. Place the baking sheet on a larger baking sheet to catch any potential drips.
Step 11: Bake the Berry Slab Pie for 20 minutes at 400°F (200°C). Then, reduce the oven temperature to 375°F (190°C) and continue baking for another 35-45 minutes, or until the crust is deeply golden brown and the filling is bubbly and thick. If the crust starts to brown too quickly, loosely tent it with aluminum foil.
Step 12: Remove the Berry Slab Pie from the oven and place it on a wire rack to cool completely before slicing and serving. Cooling allows the filling to set properly.