In a large mixing bowl, combine all-purpose flour, sugar, and salt. Cut in unsalted butter until the mixture resembles coarse crumbs. Gradually add cold water, mixing until a dough forms. Shape the dough into a disk, wrap in plastic wrap, and refrigerate for at least 2 hours.

After chilling, lightly flour your work surface and roll out two disks of dough to about 1/8 inch thickness. Use parchment paper to facilitate easy transfer to your rimmed baking sheet.

In a medium bowl, combine sliced fresh strawberries and blackberries with sugar, lemon juice, and a pinch of salt. Mix gently to coat the berries evenly while avoiding mushiness.

Roll out a second piece of dough, carefully placing it over the filled crust. Make several slits in the top crust to allow steam to escape during baking. Apply an egg wash over the top.

Preheat your oven to 375°F (190°C). Place the assembled berry slab pie in the oven and bake for 40-45 minutes, or until the crust is golden brown.

Once baked, remove the berry slab pie from the oven and allow it to cool on a wire rack for at least 30 minutes. After it has cooled, slice the pie into squares for serving.