Combine all the ingredients for the crumble in a 9x9" or 8x9" baking dish. Toss with your fingers to thoroughly combine.
Set aside while you make the topping.
For the Topping
Mix the flour, sugar, and oats in a large bowl. Add the butter, and cut in with a pastry cutter or two knives until the butter is about pea-sized. Spread the crumble evenly over the top of the filling.
Bake for 35-40 minutes, until the crumble is is golden brown and the filling is bubbling at the edges.
Notes
This topping recipe makes A LOT of topping. You can easily cut it in half if you prefer. I just happen to think the topping is the best part :)