1. Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking dish.
2. To make the shortbread base, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.
3. Add in the vanilla extract and mix until incorporated.
4. Gradually add the flour, mixing until you have a crumbly dough.
5. Press the dough firmly into the prepared baking dish, ensuring an even layer. Bake for 15 minutes or until lightly golden.
6. While the base bakes, prepare the apple filling. In a large bowl, combine the diced apples, brown sugar, cinnamon, nutmeg, and lemon juice. Toss until the apples are well coated.
7. Spread the apple mixture over the partially baked shortbread base.
8. For the crumb topping, mix together the oats, flour, brown sugar, and diced butter until the mixture resembles coarse crumbs.
9. Sprinkle the crumb topping evenly over the apples.
10. Bake for an additional 30-35 minutes, or until the top is golden brown and the apples are tender.
11. Let the bars cool completely before slicing. This helps them hold their shape when you serve them.

One tip from my experience: if you find your bars a little too crumbly to handle, try chilling them in the fridge for a bit before cutting. It’s a little trick I picked up during those hectic school mornings when I needed something quick yet satisfying for the kids.