Add the apple pieces, sugar, cinnamon and cornstarch to a large bowl and toss well. Set aside.
Add the flour, oats, cinnamon, and brown sugar to a medium bowl and whisk until combined.
Pour the melted butter over the flour mixture and toss with a fork until everything is moistened and well combined.
Pour the apple mixture into your slow cooker and even out the top with a spoon.
Spread the flour mixture over the apples evenly with a spoon.
Put the lid on the slow cooker and cook for 2 hours on high or 3 1/2 to 4 hours on low.
Serve with vanilla ice cream, whipped cream and caramel sauce or your choice of topping.