To prepare the crumble, combine flour, oats, sugars, and salt in a large bowl. Cut in the cold butter until the mixture is crumbly, then press half of it into a parchment-lined 9x13 inch baking pan for the crust.

For the apple filling, toss the diced apples with lemon juice, sugar, cinnamon, nutmeg, and cornstarch. Spread this mixture evenly over the prepared crust.

Sprinkle the remaining crumble mixture over the apple filling. Bake in a preheated oven at 375°F (190°C) for 45-50 minutes, or until the topping is golden brown and the apples are tender.

While the bars bake or cool, prepare the salted caramel. Cook granulated sugar and water in a saucepan over medium heat until it turns an amber color. Carefully whisk in the warm heavy cream and butter, then stir in the salt.

Allow the bars to cool completely in the pan before drizzling with the salted caramel sauce. Once cooled and drizzled, cut into squares and serve.