Put the rhubarb into a shallow oven dish, sprinkle with the caster sugar and shake the dish so the rhubarb pieces are in single layer. Dot the ginger slices among the rhubarb. Cover the dish with foil and bake for 10 minutes. Remove the foil and return the rhubarb to the oven for another 5 minutes or until the rhubarb is just tender. Remove the pieces of ginger and set aside.