SALTED CARAMEL DRIZZLE:
Stir the melted butter, granulated sugar, vanilla, and salt together in a medium bowl. Add the flour and stir until everything is combined. Press the mixture evenly into the prepared baking pan. Bake for 15 minutes.
Combine the sliced apples, flour, granulated sugar, cinnamon, and nutmeg together in a large bowl until all of the apples are evenly coated. Set aside.
Mix the oats, brown sugar, cinnamon, and flour together in a medium bowl. Cut in the chilled butter with a pastry blender until the mixture resembles coarse crumbs. Set aside.
Heat granulated sugar in a medium saucepan over medium heat, stirring constantly.
Sugar will melt into a thick amber-colored liquid as you continue to stir. Be careful not to burn. Once sugar is completely melted, immediately add the butter. Stir the butter into the caramel until it is completely melted, about 2-3 minutes. Very slowly, drizzle in 3/4 cup of heavy cream while stirring. Allow the mixture to boil for 1 minute. Remove from heat and stir in 1 teaspoon of salt. Allow to cool before using.