Preheat oven to 375°F (190°C). Grease and flour a 9x13 inch baking pan.

Prepare the apple filling: In a large bowl, combine the sliced apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, and salt. Toss gently to coat.

Make the salted caramel: In a medium saucepan over medium heat, combine the granulated sugar and water. Do not stir. Cook, swirling the pan occasionally, until the sugar dissolves and turns a deep amber color. Carefully pour in the heavy cream (it will bubble up vigorously!), whisking constantly until smooth. Remove from heat and stir in the butter and sea salt until melted and combined. Set aside to cool slightly.

Assemble the bars: Spread half of the apple filling evenly in the prepared pan. Pour the cooled salted caramel sauce over the apples. Top with the remaining apple filling.

Make the crumble topping: In a medium bowl, combine the flour, oats, brown sugar, cinnamon, and salt. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs.

Bake: Sprinkle the crumble topping evenly over the apple filling. Bake for 45-50 minutes, or until the topping is golden brown and the filling is bubbly.

Cool: Let the bars cool completely in the pan before cutting into squares. This allows the caramel to set.