1. Preheat your oven
Set your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, making sure there’s a bit hanging over the edges — this helps you lift the bars out easily later.

2. Make the shortbread base
In a large bowl, beat the butter, sugar, and vanilla together until creamy and smooth. Add in the flour and salt, stirring until everything comes together into a soft dough. Press the mixture evenly into your prepared pan — use the back of a measuring cup to pack it tightly for a solid base.
Bake for 15–18 minutes, or until you see a light golden hue around the edges. Set aside while you prep the apples.

3. Prepare the apple layer
In another bowl, toss the sliced apples with sugar, flour, cinnamon, and nutmeg until the apples are evenly coated in that spiced goodness. Arrange them evenly over the baked crust — try not to stack them too high; even layers bake best.

4. Make the crumble topping
In a medium bowl, combine oats, flour, brown sugar, cinnamon, and salt. Drizzle in the melted butter and stir until you get clumps — like little nuggets of delicious streusel. Sprinkle the topping generously over the apple layer, covering every inch.

5. Bake the bars
Slide the pan into the oven and bake for 30–35 minutes, or until the crumble is golden brown and the apples are soft and bubbly underneath.

6. Cool and slice
Let the bars cool completely in the pan — seriously, this step matters. The layers set as they cool, making them easier to slice neatly. Once cool, use the parchment paper overhang to lift the bars out of the pan, cut into squares, and enjoy!