1. 1. Prepare your baking dish and oven: Preheat your oven to 375 F (190 C). Lightly grease a 9×13 inch baking dish with butter or cooking spray. This prevents sticking and makes for easy removal later.

2. 2. Make the crumble topping and base: In a large bowl, combine the flour, brown sugar, cinnamon, nutmeg, and salt. Add the cold, cubed butter. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. This creates that lovely crumbly texture.

3. 3. Form the bottom crust: Press about two-thirds of the crumble mixture evenly into the bottom of the prepared baking dish. Gently pat it down to form a firm crust. This will be the base for your apple layer.

4. 4. Prepare the apple filling: In a separate medium bowl, combine the diced apples, granulated sugar, and lemon juice. Toss gently until the apples are evenly coated. The sugar will help draw out some of the apple’s natural juices, and the lemon juice adds a nice tartness.

5. 5. Assemble the layers: Spread the apple mixture evenly over the pressed crust in the baking dish. Try to get a single layer of apples so they cook consistently.

6. 6. Add the top crumble: Sprinkle the remaining one-third of the crumble mixture evenly over the apple layer. Don’t press it down; leave it loose so it can crisp up beautifully during baking.

7. 7. Bake until golden: Place the baking dish in the preheated oven and bake for 35-40 minutes, or until the topping is golden brown and the apples are tender when pierced with a fork. You should also see the apple filling bubbling around the edges.

8. 8. Cool before slicing: Remove the dish from the oven and let the bars cool completely on a wire rack before slicing and serving. This is crucial for the bars to set properly and hold their shape.