From 1922:
During hot weather, plan to have fish and meat oftentimes cold instead of hot. The following recipe is very good and hearty enough to be served as the main dish for luncheon or supper. Any fish may be used in the same way.
One two-pound mackerel, 3 cups cold water, 1 small piece bayleaf, 2 slices onion, 1 sprig parsley, 2 teaspoons of salt, 1 tablespoon vinegar, 2 tablespoons of granulated gelatine, ½ cup of cold water, 1 cup of sour cream, 2 tablespoons grated horseradish, salt to taste.
Cut off head and tail of mackerel and cook in the water with the seasonings, salt and vinegar until tender.
Take from the water, remover skin and bones and separate into small pieces. Strain the liquid remaining and add boiling water to make two cups.
Add the gelatine which has been dissolved in the cold water, season with salt and pepper and allow it to become cool.
As it begins to set add the mackerel and turn into molds which have been dipped into cold water. Set away to harden.
At serving time unmold on lettuce leaves and serve with the sour cream beaten until stiff, to which has been added the horseradish and seasoning.