Preheat and prep: Heat your oven to 350°F (175°C). Lightly butter a 9-inch square baking dish or similar 2-quart dish.
Make the caramel (if homemade): In a heavy saucepan, melt 1 cup sugar over medium heat, stirring just until it liquefies and turns deep amber.
Whisk in 6 tablespoons butter until smooth. Slowly whisk in 1/2 cup heavy cream (it will bubble). Remove from heat and stir in 1/2–3/4 teaspoon salt and 1 teaspoon vanilla.
Cool 5–10 minutes to thicken slightly.
Prep the apples: Peel, core, and slice apples 1/4-inch thick. Toss with lemon juice, 1/4 cup granulated sugar, 2 tablespoons brown sugar, 1 tablespoon flour, cinnamon, nutmeg, and a pinch of salt. Spread evenly in the baking dish.
Add caramel: Drizzle about 1/2 to 2/3 cup caramel over the apples.
Reserve the rest for serving.
Make the crumble topping: In a bowl, combine 3/4 cup flour, 3/4 cup oats, remaining brown sugar (about 6 tablespoons), and 1/4 teaspoon salt. Cut in the cold butter with a pastry cutter or your fingers until large, pea-sized clumps form. For extra crunch, mix in nuts if using.
Top and bake: Sprinkle the crumble evenly over the apples.
Bake 40–50 minutes, until the topping is golden and the apple juices are bubbling around the edges.
Rest and serve: Let the crumble rest 10–15 minutes so the juices thicken. Serve warm with ice cream and an extra drizzle of salted caramel.