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 for easy removal.
Step 2: Prepare the Crust: In a medium bowl, combine the melted butter, 1/2 cup granulated sugar, vanilla extract, and 1/4 teaspoon salt. Stir in 2 cups of all-purpose flour until a soft dough forms. Press this dough evenly into the bottom of the prepared pan. Bake for 15-18 minutes, or until the crust is lightly golden. Remove from the oven and let cool slightly while you prepare the remaining components.
Step 3: Prepare the Apple Filling: In a large bowl, combine the thinly sliced apples, 1/2 cup granulated sugar, 2 tablespoons all-purpose flour, 1 teaspoon ground cinnamon, and 1/4 teaspoon ground nutmeg. Toss gently until all the apple slices are evenly coated.
Step 4: Prepare the Streusel Topping: In another medium bowl, combine the rolled oats, brown sugar, 1/2 cup all-purpose flour, and 1/2 teaspoon ground cinnamon. Add the cold, cubed butter. Use your fingertips, a pastry blender, or a fork to cut the butter into the dry ingredients until coarse, crumbly bits form.
Step 5: Assemble the Bars: Spread the prepared apple filling evenly over the partially baked crust in the pan. Evenly sprinkle the streusel topping over the apple layer.
Step 6: Bake: Return the pan to the oven and bake for an additional 35-45 minutes, or until the apples are tender when pierced with a fork and the streusel topping is beautifully golden brown and bubbly. If the streusel begins to brown too quickly, you can loosely tent the pan with aluminum foil.
Step 7: Cool and Drizzle: Remove the pan from the oven and place it on a wire rack to cool completely at room temperature. This is a critical step for ensuring clean, neat slices. Once fully cooled, generously drizzle the salted caramel sauce over the top of the bars.
Step 8: Serve and Store: Slice the cooled bars into squares and serve. Store any leftover Salted Caramel Apple Pie Bars in an airtight container in the refrigerator for up to 3 days.