Did you know rhubarb is actually a vegetable? Well it is, and it is utterly wonderful in a crumble. Maincrop rhubarb is available in the summer and autumn, but if you're lucky, you might be able to get your hands on Yorkshire forced rhubarb in February/March - a very special treat! This recipe uses both rhubarb and apple, but you could leave out either if you wish. If you're only using rhubarb, you might want to add more sugar to the filling.