Preheat the oven to 350°F. Lightly grease a 9×13-inch baking dish with cooking spray.
Peel and slice the peaches, then place them in a large mixing bowl. Add the granulated sugar, cornstarch, and 2 tablespoons of the flour. Toss until the peaches are evenly coated, then spread the mixture into the prepared baking dish.
In a separate bowl, combine the brown sugar, remaining flour, oats, cinnamon, nutmeg, and butter. Mix until crumbly, then sprinkle the topping evenly over the peaches.
Bake for 30–40 minutes, or until the topping is golden brown and the peach filling is bubbling. Serve warm.
Notes
Use ripe, juicy peaches for the best flavor.
Don't skip the cornstarch; it creates a thick, syrupy filling.
Use cold butter to keep the topping crisp and crumbly.
Stir chopped pecans into the topping for extra crunch.