Prepare your pan and oven: Preheat your oven to 350°F (175°C). Line an 8×8 inch baking pan with parchment paper, leaving overhang on the sides for easy removal later.
Make the shortbread crust: In a medium bowl, combine melted butter, sugar, vanilla, and salt. Add flour and stir until everything is incorporated. Press the mixture evenly into the bottom of your prepared pan. Bake for 15 minutes until the edges are slightly golden.
Prepare the apple filling: While the crust bakes, toss the sliced apples with flour, lemon juice, cinnamon, nutmeg, and both sugars until all slices are evenly coated. Set aside to allow flavors to meld.
Make the streusel topping: Combine flour, brown sugar, oats, and cinnamon in a medium bowl. Add cold cubed butter and use a pastry cutter or your fingers to work the butter into the dry ingredients until you have coarse crumbs. Refrigerate until ready to use.
Assemble the bars: When the crust is done, remove from oven but keep the oven on. Layer the apple mixture evenly over the hot crust, trying to create a level surface. Sprinkle the streusel topping evenly over the apples.
Bake the assembled bars: Return the pan to the oven and bake for 30-35 minutes, until the streusel is golden brown and the apple filling is bubbling at the edges.
Make the salted caramel sauce: While the bars bake, heat sugar in a medium saucepan over medium heat, stirring constantly with a heat-resistant spatula. The sugar will form clumps before melting into a amber-colored liquid. Once completely melted, immediately add the butter (mixture will bubble rapidly). Whisk until butter is completely incorporated, about 2 minutes.
Finish the caramel: Slowly pour in the heavy cream while whisking. Allow the mixture to boil for 1 minute, then remove from heat and stir in the sea salt. Let cool slightly before using.
Cool and finish: Allow bars to cool completely in the pan on a wire rack. Once cooled, lift out using the parchment overhang and drizzle with the salted caramel sauce. For cleaner cuts, refrigerate for 1 hour before cutting into squares.