Place the flour in a bowl and add the butter. Rub the butter into the flour then stir in the muscovado sugar to make the crumble topping.
ASSEMBLE CRUMBLE:
Preheat the oven to 180C (fan 160°C)
Cut the rhubarb into 3 inch sticks and place on an oven tray. Sprinkle with 4 tablespoons of water and the caster sugar.
Roast for 10 minutes and then sprinkle the ginger and mix well.
Fill an ovenproof dish with the rhubarb.
Sprinkle the crumble topping over the rhubarb and bake for 30 minutes, or until the crumble topping is crisp and golden-brown. Also make sure that the rhubarb filling is bubbling. Test if the rhubarb is tender and if not cover the crumble with foil and leave further in the oven.
Allow to cool slightly before serving with double cream.