This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
1 salt mackerel
Clean the fish by scraping off rusty-looking portions and also the thin black membrane found on the inside, and leave it over night in plenty of cold water, with the skin side up. In the morning, drain the fish and place it in a frying-pan, skin side down, cover with fresh water, and slowly heat to the boiling-point. Drain off this water, add just enough fresh water to cover the fish and simmer until tender. Lift the mackerel out carefully (a pancake-turner will be found convenient for such work) and place it on the serving-dish in the oven to keep hot while the gravy is being prepared.
To one cup of the water left in the frying-pan after the removal of the fish, add the two cups of milk. When the liquid boils, add the flour stirred to a paste with the fat, and season with salt and pepper. Let the gravy boil slowly three or four minutes, stirring constantly until smooth. Pour it over the mackerel.
Baked - Prepare the fish as for boiling but place it in a shallow baking-pan just large enough to hold it, and pour over it the milk. Bake twenty minutes in a hot oven (400°-425° F.) stirring into the milk at the end of fifteen minutes a smooth paste made of two tablespoons of flour and two tablespoons of fat, with a sprinkling of pepper. Serve with the thickened milk poured around the fish.