Step 1: Preheat your oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment paper.
Step 2: In a large bowl, whisk together the flour, oats, granulated sugar, brown sugar, cinnamon, baking powder, and salt for the crust and crumble mixture.
Step 3: Add the very cold, cubed butter to the dry ingredients. Using a pastry blender, your fingertips, or a food processor, cut the butter into the mixture until it resembles coarse crumbs with some pea-sized pieces remaining.
Step 4: Press about 2/3 of the crumble mixture evenly into the bottom of the prepared baking pan. This will form your shortbread crust.
Step 5: Bake the crust for 10-12 minutes, or until lightly golden brown. While the crust bakes, prepare the apple filling.
Step 6: In a separate bowl, combine the peeled, cored, and diced apples with granulated sugar, flour, cinnamon, nutmeg, and lemon juice. Toss gently until the apples are evenly coated.
Step 7: Once the crust is lightly baked, remove it from the oven. Evenly spread the apple filling over the warm crust.
Step 8: Sprinkle the remaining 1/3 of the crumble mixture over the apple layer.
Step 9: Return the pan to the oven and bake for another 35-40 minutes, or until the topping is golden brown and the apples are tender when pierced with a fork.
Step 10: Once baked, remove the bars from the oven and place them on a wire rack to cool completely in the pan. This is crucial for the bars to set properly.
Step 11: Once completely cooled, gently lift the bars out of the pan using the parchment paper overhang. Drizzle generously with salted caramel sauce. If desired, sprinkle with flaky sea salt for an extra touch.
Step 12: Slice into squares and serve. Enjoy your delicious Salted Caramel Apple Crumble Bars!