Chop the rhubarb into 2.5cm/1" pieces and place them in a bowl.
Sprinkle the white sugar, ground ginger and cornflour all over and mix thoroughly using your hands. Place in the baking dish and set aside.
The Crumble
If you have a large chopper or food processor, place the cold butter and flour and pulse until the mixture resembles wet sand, then tip into a large bowl. Otherwise, chop up the butter into little cubes and rub into the flour using your fingertips. Don't knead. Do all this in a large bowl.
Add the demerara and white sugar to the flour/butter mix and using your fingertips, mix it all in. Do not overwork, keep it light, crumbly and airy.
Assembly
Sprinkle the crumble all over the rhubarb in an even layer. Don't press it down. This will leave the crumble light and crispy.
Bake for about 35-45 minutes, until the top is a golden brown, looks crispy and the sides are bubbling a little with the rhubarb. I like mine brown and crispy, so I tend to go for 45 minutes. It's a matter of taste.
Take it out of the oven and let it rest for 5 minutes, then serve as mentioned above.