How To make Mackerel with Rhubarb(Irish)

2 lb Mackerel filets
2 oz Margarine
1 ea Lg. onion, chopped
1/2 lb Rhubarb, chopped
1 x Pepper and salt
1 x Toasted breadcrumbs
1 lb Rhubarb (for the sauce)
2 tb Sugar (for the sauce)
1 x Grated lemon rind (for sauce
2 tb Water (for the sauce)
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.