Step 1: In a large bowl, whisk together the flour, sugar, and salt for the crust.
Step 2: Add the very cold, cubed butter to the flour mixture. Using a pastry blender or your fingertips, cut the butter into the flour until it resembles coarse meal with some pea-sized pieces of butter remaining.
Step 3: Gradually add ice water, one tablespoon at a time, mixing lightly with a fork or your hands until the dough just comes together. Form into a disc, wrap in plastic, and refrigerate for at least 30 minutes.
Step 4: While the dough chills, prepare the peaches. Wash, pat dry, and slice the peaches into 1/4-inch thick wedges. Peeling is optional.
Step 5: In a medium bowl, gently toss the sliced peaches with granulated sugar, cornstarch, lemon juice, and ground cinnamon (if using).
Step 6: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
Step 7: On a lightly floured surface, roll out the chilled dough into a rough 12-14 inch circle, about 1/8 inch thick. Carefully transfer the dough to your prepared baking sheet.
Step 8: Arrange the peach slices in the center of the dough, leaving a 2-inch border around the edges.
Step 9: Fold the exposed dough border over the peaches, pleating it as you go. Brush the folded crust with the egg wash and sprinkle with coarse sugar.
Step 10: Bake for 40-50 minutes, or until the crust is golden brown and the filling is bubbling. Let cool for 15-20 minutes before slicing and serving.