Preheat the oven to 180C. Cut the rhubarb into 7½cm long sticks and place on an GN container and sprinkle with 4 tablespoons of water and the caster sugar. Roast for 4 minutes. Sprinkle over the ginger and mix well. Fill the GN container 4cm deep with the rhubarb. Rub the butter into the flour and sugar to make the crumble topping. Sprinkle over the rhubarb and bake for 20 minutes, or until the crumble topping is crisp and golden-brown and the rhubarb filling has softened and is bubbling. Allow to cool slightly before serving with double cream.