Tip 500g thumb-length chunks of rhubarb into a saucepan with 100g golden caster sugar.
Cover and simmer on a very low heat for 15 mins, adding more sugar if you want. When soft (but still holding its shape) and sweet enough, pour the rhubarb into a medium baking dish.
Mix in the ginger.
Heat oven to 200C/180C fan/gas 6.
To make the topping, rub 140g self-raising flour and 85g chilled butter together with your fingers until you have a soft, crumbly topping.
Now add 50g light brown muscovado sugar and 50g chopped walnuts, if using. Mix together with your hands.
Bake the crumble topping on a baking sheet in the top of the oven until light brown.
Remove from oven, put on top of the fruit mix/in baking dish.
Rebake on the bottom shelf in the oven until piping hot.
Serve piping hot with a big jug of thick vanilla custard. Refer to the tip for a recipe.