Preheat the oven to 350°F/180°C.
Butter a cast iron 9 x 13-in. baker.
In a large mixing bowl, combine the filling ingredients and gently toss to coat all of the apple pieces.
Spread the apple filling into an even layer in the prepared pan.
In another mixing bowl, combine all topping ingredients and work the butter into the dry ingredients with your fingertips to make a crumbly mixture the size of small peas.
Cover the apples with the topping and bake 40 - 50 minutes.
Cool 15 minutes and serve warm with ice cream.