Prep the slow cooker: Lightly grease the crock with cooking spray or butter.
This helps prevent sticking and makes cleanup easier.
Prep the apples: Peel, core, and slice the apples into 1/2-inch thick slices. Try to keep them uniform so they cook evenly.
Season the fruit: In a large bowl, toss apples with lemon juice, granulated sugar, 1/3 cup brown sugar, cornstarch, cinnamon, nutmeg, and vanilla until evenly coated.
Layer in the crock: Add half the apple mixture to the crock. Scatter half the caramels (or drizzle half the caramel sauce) over the apples.
Add the remaining apples, then the rest of the caramels or sauce.
Make the crisp topping: In another bowl, stir together oats, flour, 1/2 cup brown sugar, and salt. Pour in melted butter and mix with a fork until pebble-like clumps form. Stir in nuts if using.
Add the topping: Sprinkle the oat mixture evenly over the apples, covering the surface in a loose, even layer.
Do not pack it down.
Cook low and slow: Cover and cook on Low for 3 1/2 to 4 1/2 hours, or on High for 2 to 2 1/2 hours. The apples should be tender but not mushy, and the topping should be set and lightly toasty.
Vent for crispness: In the last 20–30 minutes, prop the lid slightly with a wooden spoon to let steam escape. This helps the topping stay crisper and prevents sogginess.
Rest before serving: Turn off heat and let the crisp sit uncovered for 10–15 minutes.
The caramel thickens as it cools slightly, making it easier to scoop.
Serve: Spoon warm crisp into bowls and finish with vanilla ice cream or whipped cream. A pinch of flaky salt on top makes the caramel pop.