Heat oven to 375 F.
Slice apples as thin as the setting on you mandolin goes. This makes the perfect texture for your filling and allows the apples to cook through in time to not burn the topping.
Stir up with a little salt and cinnamon. You don't want to make the apples salty, just use a table shaker to get a little on there.
Spread filling in medium cast iron skillet or a 9x9 casserole dish. Cut up the butter and dot around the fruit.
In a medium bowl mix all the topping ingredients until they come together.
Using your hands scatter the topping mix over the filling until it is more or less evenly distributed.
Bake at 375 F for about 30 minutes. Apples should be tender with a fork and the topping should be a dark golden brown but not burned.