Make the shortbread crust:
Preheat the oven to 180°C (160°C fan-forced) / 350°F. Line an 8-inch square baking pan with baking paper.

Melt 42 g (roughly 1 ½ oz / 3 Tbsp) of the butter, then set aside to cool slightly. Keep the rest of the butter cold.

In a large bowl, whisk together the flour, both sugars, cinnamon, nutmeg, and salt. Grate the remaining cold butter using the large side of a box grater, then work it into the flour mixture with your fingertips or a pastry blender until it resembles breadcrumbs. The butter should be in roughly lentil-sized pieces.

Stir in the melted butter with a spatula and your hands until well distributed and the mixture clumps together when squeezed. Press ⅔ of the mixture into the base of the pan, then mix the oats into the remaining ⅓ for the topping and set aside.

Bake the base for 15 minutes.

Prepare the filling:
Peel and quarter the apples, then slice them thinly (about 3mm / ⅛ inch thick). You should have about 3 ½–4 cups once sliced. Combine the apples, brown sugar, flour, and cinnamon in a bowl, tossing until evenly coated.

Once the base is ready, layer the apple slices evenly over the top, arranging them to reduce gaps. Drizzle over 2 tablespoons of the caramel sauce. Scatter the remaining cookie dough (with oats) over the apples.

Bake the bars:
Bake for a further 35–40 minutes, until the top is golden brown and the apples are tender but still have a little bite when pierced with a toothpick.

Cool and serve:
Cool in the pan for at least 1 hour, or until firm enough to lift out using the paper overhang. Chill to cool completely before slicing into 9, 12, or 16 bars.

Once sliced, drizzle with slightly warmed salted caramel sauce and sprinkle with flaky sea salt (optional). Serve immediately.