Preheat the oven to 350⁰F degrees. Grease a 2-quart baking dish.
In a medium bowl stir together the flour, brown sugar, oats, cinnamon & salt.
Add the butter and cut it in using a pastry cutter or rub the butter into the dry ingredients using your fingers. It should be crumbly in texture, but no large chunks of butter should remain. Place in the fridge as you make the filling.
Peel, core and slice the apples into thin slices.
Whisk together the melted butter, flour, whiskey, sugar, cinnamon & nutmeg. Pour over the sliced apples and toss to coat.
Spoon the apples into the greased pan. Crumble the oatmeal crumble evenly over the top.
Bake in the preheated oven for 40-50 minutes, or until the top is golden brown and the apples are bubbling around the edges of the pan.
Remove from oven and cool for at least 10 minutes before serving.
Top with Salted Caramel Whiskey sauce, if desired.