Preheat the oven to 200 C, 180 Fan, Gas 6. To make the crumble topping mix the oats and flour and rub in the butter until you have a pile of small crumbs. Stir in the sugar.
Mix the rhubarb and apples in a pie or gratin dish . Dredge with the muscovado sugar and sprinkle with cinnamon. Sprinkle the crumble mixture evenly and thickly over the top.
Bake for about 30 mins or a little longer, until the top is golden brown and the juice bubbling up a little around the edges. Serve hot or warm.