01 - Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, ensuring there's an overhang for effortless bar removal.
02 - Combine the melted butter, granulated sugar, all-purpose flour, and salt in a mixing bowl until a soft dough forms. Press this dough evenly into the bottom of the prepared baking pan. Bake for 15 minutes, or until the crust is lightly golden.
03 - While the crust is baking, gently toss the diced apples with lemon juice, granulated sugar, brown sugar, ground cinnamon, ground nutmeg, and all-purpose flour until thoroughly coated.
04 - In a separate bowl, whisk together the all-purpose flour, brown sugar, ground cinnamon, and salt. Incorporate the cold, cubed butter using a fork or your fingertips until the mixture forms large, crumbly clumps.
05 - Remove the partially baked crust from the oven. Evenly spread the prepared apple mixture over the warm crust. Generously sprinkle the streusel topping over the apple layer.
06 - Return the pan to the oven and bake for 30 minutes, or until the streusel topping is golden brown and the apples are visibly bubbling. Allow the bars to cool completely in the pan.
07 - While the bars are cooling, prepare the caramel. In a small saucepan, combine the granulated sugar and water over medium heat. Cook, without stirring, until the sugar transforms into a golden amber color, approximately 5–7 minutes. Remove from heat and cautiously whisk in the butter, followed by the heavy cream and a pinch of sea salt. Allow this caramel to cool slightly.
08 - Once the bars have cooled, carefully lift them out of the pan using the parchment paper overhang. Cut into individual squares and generously drizzle with the prepared caramel before serving.