1. Preheat your oven to 375 F (190 C). Lightly grease a 9×13-inch baking dish with butter or cooking spray.

2. In a large bowl, whisk together the flour, granulated sugar, cinnamon, nutmeg, and salt.

3. Add the cold, cubed butter to the dry ingredients. Using your fingertips, a pastry blender, or a fork, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces remaining.

4. Press about two-thirds of the crumble mixture evenly into the bottom of the prepared baking dish. Use the back of a spoon or your hands to compact it firmly.

5. In a separate medium bowl, combine the diced apples, lemon juice, and brown sugar. Toss gently until the apples are well coated.

6. Spread the apple mixture evenly over the pressed crust in the baking dish.

7. To the remaining one-third of the crumble mixture, add the rolled oats and mix them in. Sprinkle this oat-crumble mixture evenly over the apple layer.

8. Place the baking dish in the preheated oven and bake for 35-40 minutes, or until the topping is golden brown and the apple filling is bubbly and tender when pierced with a fork. Let the bars cool completely on a wire rack before slicing.