In a food processor, pulse flour, sugar, and salt to combine. Add cold butter and pulse until mixture resembles coarse meal with some pea-sized pieces remaining, about 10 to 12 pulses.

Drizzle 4 tablespoons ice water over mixture and pulse just until dough starts to come together, 3 to 4 pulses. Squeeze a small handful - it should hold together without crumbling. If too dry, add more water 1 teaspoon at a time.

Turn dough onto a lightly floured surface and pat into a 1-inch thick disk. Wrap tightly in plastic and refrigerate at least 1 hour or up to 2 days.

In a large bowl, combine peach slices, brown sugar, cornstarch, cinnamon, and lemon juice. Toss gently until evenly coated. Let stand 10 minutes to release juices.

On a floured sheet of parchment paper, roll chilled dough into a rough 12-inch circle about 1/8 inch thick. Slide parchment and dough onto your baking sheet.

Drain excess liquid from peaches (save for drizzling later). Mound fruit in center of dough, leaving a 2-inch border. Fold border up and over fruit, pleating loosely as you go - perfection is not the goal here.

Brush folded crust with egg wash and sprinkle with turbinado sugar. Drizzle reserved peach juices over exposed fruit. Bake at 375F until crust is deep golden and fruit is bubbling, 40 to 45 minutes. Cool on sheet 15 minutes before slicing.