Preheat oven to 180°C. Mix the filling ingredients into a bowl and mix well to combine. Transfer the filling to a large dish – large enough to leave room for the crumble topping.
Put the the flour, sugar, cinnamon, oats and butter for the crumble topping in a large bowl and mix until combined together. No need to over-mix!
Spread the crumble mixture over the filling in the dish. Bake for 40–45 minutes, or until the crumble is toasted and the apples are soft.