Preheat your oven to 350°F (175°C) and lightly grease a 9×13 inch baking dish.
Make the caramel sauce first by heating the sugar in a medium saucepan over medium heat, stirring constantly with a heat-resistant spatula until it melts and turns amber (about 5-7 minutes). Be patient and watch carefully as sugar can burn quickly.
Add the butter to the melted sugar carefully as it will bubble vigorously. Stir until completely melted and incorporated (about 2-3 minutes).
Slowly pour in the heavy cream while stirring constantly. The mixture will bubble aggressively again – this is normal. Continue stirring until smooth (about 1 minute).
Remove from heat and stir in vanilla and salt. Transfer to a heat-safe container and set aside to cool slightly while preparing the rest of the crisp.
Prepare the apple filling by combining the sliced apples and lemon juice in a large bowl, tossing to coat. Add sugar, flour, cinnamon, nutmeg, and salt, mixing until apples are evenly coated.
Make the crisp topping by combining oats, flour, brown sugar, pecans (if using), cinnamon, and salt in a medium bowl. Add cold butter cubes and use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
Layer your crisp by arranging the apple mixture in the prepared baking dish. Drizzle about ⅔ of the caramel sauce evenly over the apples (reserve the remaining caramel for serving).
Sprinkle the crisp topping evenly over the apple and caramel layers, covering completely.
Bake for 45-55 minutes until the topping is golden brown and the filling is bubbling around the edges. If the topping browns too quickly, loosely cover with aluminum foil.
Allow to cool for 15-20 minutes before serving. This cooling time helps the filling set and prevents burning your mouth on molten caramel.
Serve warm with a scoop of vanilla ice cream and a drizzle of the reserved caramel sauce.