Preheat your oven to180°c (160°c for fan-assisted ovens, Gas Mark 5 or 350°F).
Chop your rhubarb into thumb-sized pieces. Place your chopped rhubarb in your dish, making sure it covers the bottom and you've no big gapping spaces – you want it fairly well compacted.
Sprinkle the 2 tbsp of caster sugar over the rhubarb, before pouring your orange juice over it. Set aside while you prepare your crumble mixture.
In a large bowl, add the flour and butter/margarine, and using your fingertips gently rub them together until you have a rough sandy texture. Stir through the caster sugar.
Spoon the crumble mixture over your rhubarb, being sure to cover all the lumps and bumps. Using the tips of your fingers, gently press the top of the crumble mixture, to help compact some of it down but whilst leaving a bumpy texture to the top.
Bake your crumble in your pre-heated oven for 35-40 minutes, or until golden on top.
Allow your crumble to cool a little before serving. Enjoy with custard or ice cream, or just on it's own.