Wash and top and tail the rhubarb and cut the stalks into 5cm pieces. Zest and squeeze the orange.
Put the rhubarb and orange zest into a saucepan and drizzle with the orange juice. Cover the pan with a lid and cook over a low heat for about 7-8 minutes or until the rhubarb pieces collapse into a mash.
Meanwhile, blitz the flour, oats, butter and sugar together to the consistency of bread crumbs.
Arrange the rhubarb in a 2-litre ovenproof dish and top with the crumble. Bake for 30 minutes or until golden brown on top.
Serve hot or warm with homemade vanilla custard.
Just 20 minutes easy prep then half an hour in the oven while you get on with something else.