Preheat the oven to 350 degrees F. Lightly butter a 9-by-13-inch baking dish.
Add the apples to a large bowl. Melt the butter in a medium saucepan set over medium heat. Stir in the brown sugar, heavy cream, vanilla, cinnamon, nutmeg, and salt. Cook for 2-3 minutes, stirring, until the sugar has dissolved and the mixture is smooth. Whisk in the 2 tablespoons of flour and cook for approximately 1 minute, until slightly thickened.
8 cups cored, and sliced apples, 1/4 cup unsalted butter, 1/2 cup packed light brown sugar, 1/4 cup heavy cream, 1 teaspoon pure vanilla extract, 1 teaspoon ground cinnamon, 1/4 teaspoon ground nutmeg, 1/4 teaspoon fine sea salt, 2 tablespoons all-purpose flour
Pour the caramel mixture over the apples and toss until evenly coated. Transfer the apples to the prepared baking dish and spread them into an even layer.
To make the topping, combine the oats, flour, brown sugar, pecans, cinnamon, and salt in a medium bowl. Add the melted butter and stir until the mixture forms moist, crumbly clusters.
1 cup old-fashioned rolled oats, 3/4 cup all-purpose flour, 1/2 cup packed light brown sugar, 1/2 cup chopped pecans, 1/2 teaspoon ground cinnamon, 1/4 teaspoon fine sea salt, 6 tablespoons unsalted butter
Sprinkle the topping evenly over the apples. Bake for 45-50 minutes, until the apples are tender and bubbling around the edges and the topping is golden brown.
While the crisp is baking, prepare the caramel sauce: Combine the sugar and water in a medium saucepan. Cook over medium heat, without stirring, until the sugar dissolves and the syrup turns a deep amber color, about 7-9 minutes. Swirl the pan occasionally if needed to encourage even browning.
1 cup granulated sugar, 1/4 cup water
Remove the pan from the heat and carefully whisk in the butter, a few pieces at a time. Be careful; the mixture will bubble vigorously.
4 tablespoons unsalted butter
Slowly whisk in the warm heavy cream until smooth. If the caramel seizes slightly, return the saucepan to low heat and whisk until smooth.
1/2 cup heavy cream
Stir in the vanilla and salt. Let the caramel sauce cool for 5-10 minutes.
1 teaspoon pure vanilla extract, 1/2 teaspoon fine sea salt
Once the apple crisp has finished baking, let it cool for 10-15 minutes before serving. Serve warm with vanilla ice cream or whipped cream and drizzled with caramel sauce.