Preheat the oven to 350°F (177°C). Spray a 9-inch pie plate (or an 8 or 9-inch square baking dish) with nonstick cooking spray and set aside.
To make the topping:
In a large mixing bowl, whisk together the flour, brown sugar, cinnamon, and salt until well combined. Add the cold cubed butter and using a pastry cutter or fork, cut the butter into the dry ingredients until the mixture starts to come together and is crumbly. Stir in the oats and transfer to the refrigerator while you make the filling.
To make the filling:
Add the sliced peaches to a large mixing bowl and sprinkle the flour and sugar on top. Stir until all of the peaches are fully coated. Scoop the peaches into the prepared baking dish, remove the topping from the refrigerator and sprinkle on top of the filling.
Bake for 40 to 50 minutes or until the topping is lightly golden brown and the juices are bubbling around the edges.
Remove from the oven and transfer to a wire rack to cool slightly before serving.
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Notes
Once the peach crisp has cooled completely, cover tightly, and store in the refrigerator for 2 to 3 days.To reheat the crisp: You may reheat individual servings in the microwave for 30 seconds or until warmed through. To reheat the full peach crisp, preheat oven to 300°F (150°C), remove the crisp from the refrigerator and let it sit at room temperature while the oven preheats, then bake in the oven for 15 to 20 minutes or until warmed through. Oats: I prefer to use old-fashioned rolled oats, but quick oats will be fine too. Peaches: You may also use frozen or canned peaches for this recipe. If using frozen peaches, thaw them first, and drain any excess juice. If using canned peaches, I suggest using peaches that are canned in juice (not syrup) and completely draining them first.