Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment, leaving overhang for easy lifting.

Make the crust: In a bowl, whisk together flour, oats, brown sugar, cinnamon, and salt. Stir in the melted butter until a crumbly mixture forms that holds when pressed.

Press the crust evenly into the pan to form a compact base. Bake 12–15 minutes, just until the edges start to golden and the crust is set. Remove from oven and let cool slightly.

Meanwhile, prepare the filling: In a large bowl, toss apples with lemon juice, granulated sugar, 1 tablespoon flour, cinnamon, and a pinch of salt until evenly coated.

Spread the apple filling in an even layer over the warm crust. Return the pan to the oven and bake 20–25 minutes, or until the apples are tender and the filling looks glossy.

Drizzle caramel sauce in a thin, even line over the hot filling. If you prefer a more dramatic swirl, drag a spoon through the caramel to create ribbons. Bake 2–3 minutes more to set the caramel, then remove from oven and cool completely before cutting.

Chill the bars for at least 1 hour for clean slices, then lift from the pan using the parchment overhang. Cut into squares or bars and serve at room temperature or slightly warmed.