How to Make It
Place peach slices in a large mixing bowl and toss to coat in the vanilla extract. In a separate bowl, whisk together sugar, cornstarch, and 2 tablespoons all-purpose flour. Dump sugar mixture onto peaches, and toss the peaches until they are evenly coated. Set aside for 30 minutes.
In the bowl of a stand mixer, combine remaining 1 cup flour, oats, brown sugar, cinnamon, and salt. Mix to combine. With the mixer on low, add one cube of butter at a time, waiting until it is incorporated before adding the next. Continue until all of the butter is used. Add pecans and mix until evenly incorporated.
Preheat oven to 350°F. Grease a glass 9- x- 13-inch baking dish with cooking spray. Spread peaches evenly inside the dish. Sprinkle oat mixture on top of the peaches, making an even layer. Bake until the peach liquid is bubbling and the surface has browned, about 1 hour. Allow to cool slightly before serving with vanilla ice cream, if desired.