Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper. In a mixing bowl, combine flour, sugar, and salt. Cut in butter until the mixture resembles coarse crumbs. Press the dough firmly into the bottom of the pan.

Peel, core, and slice apples into thin wedges. In a bowl, toss the apple slices with sugar and cinnamon. Spread the spiced apple filling evenly over the shortbread crust.

In another bowl, mix rolled oats, brown sugar, and melted butter until clumpy. Sprinkle this crumble topping generously over the apple filling.

Bake for 30–35 minutes until the topping turns golden brown and the apple filling is bubbly.

Remove from oven and cool in the pan for 15 minutes. Use the parchment to lift the bars out, cool completely, then cut into squares or bars.