Preheat your oven to 375°F (190°C). Grease and flour an 8x8 inch baking pan or line it with parchment paper with an overhang.

In a large bowl, whisk together the flour, rolled oats, brown sugar, cinnamon, and salt for the crumble mixture.

Cut in the cold, cubed butter using a pastry blender, fork, or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces.

Reserve about 1.5 cups of the crumble mixture for the topping. Press the remaining crumble mixture evenly into the bottom of the prepared baking pan to form the crust.

In a separate medium bowl, gently toss the peaches with granulated sugar, cornstarch, lemon juice, and vanilla extract until the peaches are evenly coated.

Spread the peach filling evenly over the pressed crumble crust.

Sprinkle the reserved crumble mixture evenly over the peach filling.

Bake for 35-45 minutes, or until the topping is golden brown and the peach filling is bubbling. If the topping starts to brown too quickly, you can loosely tent the pan with foil.

Let the bars cool completely in the pan on a wire rack before cutting into 12 bars. This is crucial for clean cuts and to allow the filling to set.