Preheat the oven to 180C/350F/Gas 4.
Cut the rhubarb into 7½cm/3in long sticks and place on an oven tray, sprinkle with the water and caster sugar and roast in the oven for 10 minutes.
Once cooked, remove from the oven, sprinkle over the ginger and mix well.
Fill an ovenproof dish about 4cm/1½in deep with the rhubarb.
Rub the butter into the flour and sugar to make the crumble topping. Sprinkle over the rhubarb and bake in the oven for 35-45 minutes, or until the crumble topping is crisp and golden-brown and the rhubarb filling has softened and is bubbling.
Remove and allow to cool slightly before serving with double cream.