Preheat your oven to 350°F (175°C). Line an 8×8-inch or 9×9-inch square baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease any exposed pan surfaces with butter or non-stick spray.

In a large mixing bowl, combine the all-purpose flour, rolled oats, brown sugar, one teaspoon of cinnamon, and salt. Use a pastry cutter or your clean fingertips to work the cold, cubed butter into the dry ingredients. Continue until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

Measure out approximately half of the crumble mixture (about 2 to 2.5 cups) and pour it into the prepared baking pan. Press the mixture firmly and evenly into the bottom of the pan to form a solid base. Set the remaining crumble mixture aside for the topping.

In another bowl, toss the diced apples with the lemon juice to prevent browning. Add the granulated sugar, cornstarch, one teaspoon of cinnamon, and nutmeg. Stir well until all the apple pieces are evenly coated in the sugar and spice mixture.

Spread the apple mixture evenly over the pressed bottom crust. Take the remaining crumble mixture and sprinkle it generously over the top of the apples, ensuring you cover the fruit as much as possible. Lightly press the topping down with your hands so it adheres to the apples.

Place the pan in the center of the preheated oven. Bake for 40 to 45 minutes, or until the topping is golden brown and the apple filling is bubbling around the edges. If the topping browns too quickly, you can loosely cover it with foil for the last 10 minutes.

Remove the pan from the oven and place it on a wire rack. It is crucial to let the bars cool completely in the pan—usually about 2 hours—before slicing. This allows the filling to set so the bars hold their shape. Once cooled, lift them out using the parchment paper and cut into 12 squares.