Melt the margarine (or butter) and cook the onion in it until transparent. Add the chopped rhubarb, season with pepper and salt and continue to cook gently for 5 minutes. Then add breadcrumbs and stir the mixture.  . Now lay the mackerel filets out flat, skin side down, and spread the stuffing on them. Roll each up, put them in a greased oven dish and cook them in a moderate oven (400F) for 15-20 minutes.  . While the fish are being cooked, make the rhubarb sauce by placing all the ingredients listed in a saucepan and stewing tehm until the rhubarb is cooked and quite soft. This will take 10 minutes or a little longer. Then put the cooked rhubarb through a fine sieve or the blender, to make a puree of it. This can be served either hot or cold with the cooked mackerel.