Peel, core, and slice the apples into thin ⅛-inch slices. In a large bowl, toss the apple slices with brown sugar, cinnamon, nutmeg, and flour until well coated. Set aside.
In another bowl, stir together the oats, flour, cinnamon, and brown sugar. Add the cold, cubed butter. Using your hands or a pastry cutter, work the butter into the mixture until it resembles coarse crumbs.
Spread the prepared apple mixture evenly over the pre-baked crust. Sprinkle the oat topping evenly over the apples.
Bake for 30–35 minutes, or until the top is golden brown and the apples are tender. If the topping begins to brown too quickly, loosely cover with foil during the last 10 minutes.
Let the bars cool completely in the pan on a wire rack. Once cooled, transfer the pan to the refrigerator to set. When ready to serve, lift the bars out using the parchment overhang. Drizzle generously with salted caramel sauce before slicing.