Preheat & Prep:- Preheat oven to 350°F (177°C). Line an 8×8 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal. Set aside.
Make the Oat Base:-In a large bowl, whisk together oats, flour, brown sugar, baking soda, cinnamon, and salt. Pour in the melted butter and stir until the mixture is evenly crumbly and clumps when pressed. Reserve 1 cup of this mixture for the crumb topping. Press the remaining mixture firmly into the bottom of your prepared pan using your hands or the flat bottom of a glass.
Pre-Bake the Crust:- Bake the crust for 12 minutes until edges just begin to turn golden. Remove from oven and set aside. Keep the oven on at 350°F.
Make the Apple Filling:- While the crust pre-bakes, toss the thinly sliced apples with sugar, flour, cinnamon, nutmeg, and lemon juice in a medium bowl until every slice is well coated.
Assemble the Layers:- Spread the apple filling evenly over the warm pre-baked crust. Drizzle the caramel sauce generously and evenly over the apple layer. Sprinkle the reserved oat crumble over the top and press very lightly so it adheres to the caramel. Use your fingers to form a few larger clumps for extra crunchy texture on top.
Bake:- Bake for 35–40 minutes until the crumb topping is deep golden brown and the caramel apple filling is visibly bubbling around the edges.
Cool & Slice:- Place pan on a wire rack. Cool completely for at least 1–2 hours at room temperature, or 30–45 minutes in the fridge. Once fully cooled, lift out using parchment paper overhang. Slice into 9 large bars or 16 smaller squares. Sprinkle with flaky sea salt before serving if desired.
