Preheat & Prep the Pan: Preheat your oven to 350°F (175°C). Line a 9x13-inch metal baking pan with parchment paper, leaving generous overhang on the sides for easy lifting later. Lightly coat with nonstick cooking spray.

Make & Bake the Shortbread Crust: In a large bowl or food processor, combine 2 cups flour, powdered sugar, and salt. Cut in the cold cubed butter using a pastry cutter or pulse until the mixture resembles fine, crumbly sand. Press the mixture firmly and evenly across the bottom of the prepared pan. Bake for 15 to 18 minutes until lightly set and pale golden around the edges.

Toss the Apple Filling: While the crust bakes, combine sliced apples, 2 tablespoons flour, 2 tablespoons sugar, 1.5 teaspoons cinnamon, nutmeg, and lemon juice in a large bowl. Toss thoroughly until every apple slice is evenly coated in spiced sugar.

Mix the Oat Streusel: In a medium bowl, combine rolled oats, brown sugar, 0.5 cup flour, and 1 teaspoon cinnamon. Pour in the melted butter and stir with a fork until coarse, crumbly clusters form.

Layer & Bake to Golden Perfection: Spread the apple mixture evenly over the warm par-baked crust. Scatter the oat streusel mixture generously across the apples, ensuring an even blanket of crunchy topping. Bake at 350°F (175°C) for 32 to 38 minutes, until the streusel is deep golden-brown and the apples are tender when pierced with a toothpick.

Cool, Caramel Drizzle & Slice: Transfer pan to a wire rack and allow to cool completely at room temperature (approx. 1.5 to 2 hours), or chill for 1 hour to ensure razor-clean slices. Warm the salted caramel sauce and drizzle generously across the top. Lift the bars out using the parchment overhang, slice into 16 neat squares with a sharp chef knife, and serve!