First cube the butter and then put it in the fridge to make sure it doesn't go soft. We need it to be as chilled as possible!
Chop the rhubarb so they are about the length of your thumb.
Put the rhubarb into a saucepan and add the golden caster sugar.
Add 3 tablespoons of water and simmer on a low heat for 10 minutes with the lid on. Keep an eye on it as the rhubarb should be soft but still be holding its shape.
While the rhubarb is cooking, take the butter out from the fridge and add it to a bowl with the sieved flour and the light brown sugar.
Rub between your fingers until you get what looks like breadcrumbs. Nearly there! TIP: Work quickly and make sure your hands are cold to avoid melting the better.
Done! Shake the bowl to bring the larger crumbs to the top and rub them until they're a finer consistency. Leave in the fridge to keep chilled while the rhubarb simmers.
The rhubarb is ready!
Pour the rhubarb into a baking dish and sprinkle the crumble topping over it. Place in the oven at 180C fan (200C conventional) for half an hour.
Out the oven!