Preheat your oven to 300°F (150°C). Line an 8x8-inch or 9x9-inch metal baking pan with parchment paper, leaving an overhang on the sides for easy lifting.

Prepare Shortbread Crust: In a medium bowl, whisk together melted butter, granulated sugar, vanilla extract, and salt. Stir in flour until a soft dough forms. Press evenly into the bottom of the prepared pan. Bake for 15 minutes at 300°F, then remove and increase oven temperature to 350°F (175°C).

Toss Apple Filling: In a separate bowl, toss finely diced apples with flour, brown sugar, cinnamon, nutmeg, and lemon juice until evenly coated.

Make Streusel: In another bowl, combine rolled oats, brown sugar, flour, and cinnamon. Cut in cold cubed butter using a pastry blender or your fingertips until coarse, pea-sized crumbs form.

Assemble & Bake: Layer the spiced apples evenly over the warm par-baked crust. Scatter the oat streusel generously over top. Bake at 350°F (175°C) for 30 to 35 minutes until the streusel is deep golden brown and apples are tender when poked with a paring knife.

Chill, Drizzle & Slice: Allow the bars to cool completely at room temperature for 1 hour, then refrigerate for at least 1 hour to set cleanly. Drizzle generously with warm salted caramel sauce, sprinkle with flaky sea salt, lift using the parchment handles, and slice into 16 neat squares.