Spray a 15x10x1 inch cookie sheet with non-stick cooking spray. In a large bowl combine flour and salt. Using a pastry blender cut in the cold butter cubes until it resembles coarse crumbs. Add 1 tablespoon of the cold water or buttermilk into the flour mixture and stir with a fork. Repeat using 1 tablespoon of cold water or buttermilk at a time until it is comes together. Press into a ball.
Preheat oven to 375 degrees. On a lightly floured surface, roll dough into a 19x13-inch rectangle. Wrap the dough around the rolling pin and unroll it into the prepared baking pan. Carefully press the dough up the sides of the pan.
In a large bowl, toss sliced and peeled apples, brown sugar, cinnamon, and flour together. Pour over prepared crust.
Brown Sugar Oat Topping:
In a medium-sized bowl, add butter, brown sugar, rolled oats, flour, and cinnamon together. Using a fork, mash the butter into the flour and oats until it resembles coarse crumbs and it is all mixed together. Sprinkle on top of the apples. Bake for about 35 minutes and then lightly place a large piece of aluminum foil over the pie to prevent it from becoming too brown. Cook for 10 minutes longer or until apples are softened.
Salted Caramel:
Heat a heavy-bottomed pot over medium-low heat. Add butter and sugar and stir together. Let butter and sugar melt and begin to turn a light golden color. Remove from heat and carefully add the heavy cream. It is hot and may splatter so be careful. Stir in sea salt and vanilla.
Once apple slab pie is done, drizzle salted caramel all over the pie. Serve with vanilla bean ice cream.