I sometimes slice rhubarb into about 1 inch chunks, add a bit of brown sugar to taste, a couple of tablespoons of orange juice and a bit of ground ginger.
Crumble is:
6 oz plain or wholemeal flour(or a mixture of both)
3 oz marg
3 oz sugar(brown or white, brown makes it nice and crunchy)
Rub flour and marg together then stir in sugar, spread on top of rhubarb and bake at about 170/180c, depends on your oven, gas 4 for about 35-40 mins. This would serve 4.
Mmmmm..... lovely with ice cream, custard or cream.