Preheat the oven to 180 degrees C. Grease an ovenproof dish with butter.
Fruit: Put the apples and rhubarb in the ovenproof dish and dot with butter. Mix the sugar and cinnamon together and sprinkle over the fruit.
Topping: Put the butter, flour, cinnamon and half the sugar in a mixing bowl. Rub together until the mixture resembles coarse breadcrumbs. Heap the crumble over the fruit and sprinkle with the remaining sugar.
To Cook: Bake for 45 minutes or until the top of the crumble is golden and crisp.