Step 1: Preheat your oven to 375°F (190°C) and lightly grease a 9×13 inch baking dish.
Step 2: In a large bowl, toss the sliced apples with lemon juice, granulated sugar, and cornstarch until evenly coated.
Step 3: In a small saucepan over medium heat, melt 4 tablespoons of butter with 1/2 cup of brown sugar and the heavy cream. Whisk constantly until the mixture thickens into a caramel sauce, then stir in the vanilla and sea salt.
Step 4: Pour the apple mixture into the prepared baking dish and drizzle the warm caramel sauce evenly over the fruit.
Step 5: In another bowl, combine the oats, flour, remaining brown sugar, and cinnamon. Use a pastry cutter to work in the remaining cold butter until the mixture is crumbly.
Step 6: Sprinkle the oat topping generously over the caramel-covered apples.
Step 7: Bake for 40-45 minutes, or until the topping is golden brown and the apple juices are bubbling around the edges.
Step 8: Allow the crisp to cool for at least 15 minutes before serving to let the caramel sauce set.