Prepare the pie crust: In a large bowl, whisk together the flour and salt to combine. Add the butter, tossing the cubes in the flour to coat. Rub the butter into the flour until it is the size of walnut halves
Make a well in the center, and add the water a few tablespoons at a time and mix just until the dough comes together. Wrap the dough in plastic and refrigerate for at least 1 hour and up to 3 days (or freeze up to 3 months).
Prepare the filling: While the pie dough chills, mix the berries, sugar, cornstarch, and lemon juice together in a large bowl. Cover tightly and let sit until the dough is ready.
Preheat oven to 425°F (190°C). Line a large baking sheet with parchment paper. Set aside.
On a lightly floured work surface, roll the dough into a 12-inch circle (or any shape, really!). Trim the rim of the circle to make a clean cut if desired. Transfer dough to the prepared baking sheet.
Spoon the berries (not the juices) into the center of the dough, leaving a 2-3 inch border all around. Gently fold the edges of the dough over the fruit, overlapping the dough as necessary. Press gently to seal the edges. Brush the edges with egg wash and sprinkle with coarse sugar.
Bake until the filling is bubbly and the crust is golden brown, about 25-30 minutes. Allow to cool on the baking sheet for 10 minutes before slicing and serving.