Preheat the oven to 350°F (175°C). In a large mixing bowl, combine all-purpose flour, granulated sugar, brown sugar, and cold butter. Mix until mixture resembles coarse crumbs. Set aside 1 cup for topping. Press remaining mixture into a greased 9x13-inch baking dish to form the crust.

Bake the prepared crust in the preheated oven for 10-15 minutes until lightly golden. Remove from oven and let cool slightly.

While the crust is cooling, wash and slice your fresh peaches. Toss with cornstarch, lemon juice, and optional cinnamon until well coated.

Spread the peach filling evenly over the cooled crust. Sprinkle the reserved crumb mixture over the peach filling.

Bake for approximately 35-50 minutes until the topping is golden brown and the filling is bubbling. If needed, cover with foil after 25 minutes to prevent burning.

Remove from the oven and cool at room temperature for at least 1 hour before slicing.