1 and 1/4 cups all-purpose flour, plus more for rolling
1/4 teaspoons salt
2 teaspoons granulated sugar
1/2 cup (1 stick) cold unsalted butter, cut into small pieces
3 pounds peaches, halved, pitted, and cut into 1/2-inch slices
2 tablespoons light-brown sugar
2 tablespoons all-purpose flour
1/3 cup packed light-brown sugar
1/3 cup all-purpose flour
1/3 cup old-fashioned rolled oats
6 tablespoons cold unsalted butter, cut into small pieces
Make the crust: In a food processor, pulse flour, salt, and granulated sugar until combined. Add butter; pulse until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. Sprinkle with 3 tablespoons ice water. Pulse until dough is crumbly but holds together when squeezed (if necessary, add up to 2 tablespoons ice water, 1 tablespoon at a time); do not overmix. Form dough into a disk, wrap tightly in plastic, and refrigerate until firm, at least 1 hour (or up to overnight).
Preheat oven to 375 degrees, Lightly flour a rolling pin and work surface and roll out dough to a 12-inch round. Place in a 9-inch pie plate, fold overhang under, and crimp edges.
Make filling: In a large bowl, toss together peaches, brown sugar, and flour until combined.
Make crumble: In a small bowl, combine brown sugar, flour, and oats; using your hands, work in butter until large clumps form.
Transfer peach filling to pie shell, then sprinkle crumble evenly over top.
Place pie on a rimmed baking sheet and bake until juices are bubbling and topping is golden, 1 hour to 1 hour and 15 minutes.
Let cool on a wire rack 1 and 1/2 hours before serving.