Make the sauce first by chopping the rhubarb into smallish chunks and putting it into a thick-based saucepan, adding the sugar, ginger and water.
Then put a lid on and let it cook over a fairly gentle heat, shaking the saucepan from time to time. It will probably take around 15 – 20 minutes to soften.
Now rub the contents of the saucepan through a sieve, or purée it in an electric blender, and taste to check that there is enough sugar (although it should be a fairly sharp sauce).
Now preheat the grill, wipe the mackerel as dry as possible with kitchen paper, season the insides of the fish, then make three diagonal slashes across the backbone of each fish and grill them for about 4-6 minutes on each side.
While they’re grilling, reheat the sauce and serve it with the fish, together with new potatoes.