Combine the flour, rolled oats, brown sugar, cinnamon, and salt in a large bowl. Pour in the melted butter and stir until a crumbly mixture forms.

Press about two-thirds of the crumble mixture firmly and evenly into the prepared pan. Reserve the remaining mixture for the topping.

Bake the crumble base for 10 minutes, then remove it from the oven and allow it to cool slightly.

Toss the diced apples with granulated sugar, cinnamon, cornstarch, and lemon juice until evenly coated. Spread the apple mixture over the baked base.

Melt the granulated sugar in a saucepan over medium heat, stirring carefully until it reaches a deep amber color. Remove from the heat and whisk in the butter, followed by the heavy cream. Stir in the sea salt until smooth.

Let the caramel cool for a few minutes, then drizzle approximately half of it over the apple filling.

Sprinkle the reserved crumble evenly over the apples. Bake for 25 to 30 minutes, until the topping is golden and the apple filling is bubbling around the edges.

Allow the bars to cool completely in the pan before lifting them out and slicing into 16 squares.

Drizzle with the remaining salted caramel and finish with a small amount of flaky sea salt, if desired.

Granny Smith, Honeycrisp, and Braeburn apples work particularly well because they hold their shape during baking.

Let the bars cool completely before slicing to prevent them from falling apart.

Store leftovers in an airtight container in the refrigerator for up to 5 days.

The bars can be frozen for up to 2 months.

A thick store-bought salted caramel sauce can be used when you want a quicker version.