01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing for overhang on the sides for easy removal.
02 - In a large bowl, thoroughly combine flour, oats, brown sugar, granulated sugar, cinnamon, and salt. Stir in the melted butter until the mixture becomes crumbly and even.
03 - Firmly press two-thirds of the crumble mixture into the bottom of the prepared pan to create a uniform crust. Reserve the remaining third for the topping.
04 - In a separate bowl, toss diced apples with granulated sugar, cinnamon, nutmeg, flour, and lemon juice until evenly coated.
05 - Evenly spread the apple mixture across the crust in the baking pan.
06 - Sprinkle the reserved crumble mixture evenly over the apple layer.
07 - Bake for 35 to 40 minutes until the topping is golden and apples are tender. Remove from oven and allow to cool completely in the pan.
08 - Add granulated sugar and water to a small saucepan over medium heat. Stir until sugar dissolves, then cook without stirring until the mixture darkens to amber, about 5 to 7 minutes. Remove from heat, whisk in butter and heavy cream carefully, and stir in flaky sea salt. Cool slightly.
09 - Lift cooled bars out of the pan using the parchment overhang. Generously drizzle with salted caramel, cut into bars, and serve.