Preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving overhang for easy removal.
Make the crust and topping:
In a large mixing bowl, stir together the melted butter, flour, oats, brown sugar, baking soda, and salt until the mixture forms a crumbly texture.
Press about two-thirds of this mixture evenly into the bottom of the prepared pan to form the crust. Reserve the remaining third for the topping.
Prepare the apple filling:
In a separate bowl, toss the thinly sliced apples with granulated sugar, flour, cinnamon, and nutmeg until evenly coated.
Spread the apple mixture evenly over the crust in the baking pan.
Assemble the layers:
Sprinkle the reserved oat mixture evenly over the apples.
Drizzle about half of the caramel sauce over the top.
Bake: Bake for 40–45 minutes, or until the topping is golden and the apples are soft and bubbling.
Cool and finish: Let the bars cool in the pan for 15 minutes. Drizzle the remaining caramel sauce over the top and lightly sprinkle with flaky sea salt.
Chill before cutting: For clean slices, refrigerate for at least 1–2 hours until fully set. Lift out with parchment and slice into bars.
