Directions:
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. Add breadcrumbs and stir the mixture. 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.
Sauce:
Place the ingredients in a saucepan and stew the rhubarb for around 10 minutes or more until it is cooked (soft). 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 mackerel.