Preheat oven to 350 degrees F.
Add 1 tablespoon of butter to a 10 inch cast iron skillet. Place skillet in the oven to melt the butter. Remove the skillet from the oven and swirl butter around to coat the bottom.
Peel, core, and chop the apples into 1-inch pieces. Add to a large mixing bowl.
Pour ¼ cup of bourbon caramel sauce, ½ teaspoon of cinnamon, and juice of half a lemon on top of the apples. Stir to coat the apples evenly.
Scoop apples out into the prepared skillet.
In the base of a food processor, combine cold butter, brown sugar, flour, and salt. Pulse until it forms a medium-fine crumb. (If you don't have a food processor, you can also cut the butter into the flour and sugar with your hands, a pastry cutter, or knife.)
Add in the old fashioned oats and stir with a spoon to combine. Add the oat crumble on top of the diced apples. It will be heaping, but make sure to add it all on top for a thick and crispy crust.
Place the cast iron in the oven and bake for 40 to 50 minutes, or until the apples are soft on the inside and the top is crisp and golden brown.
Allow to cool slightly before scooping, topping with vanilla ice cream or whipped cream, and drizzling extra bourbon caramel sauce on top.