Take fish. Remove skin and bones from the fish and chop flesh very fine, add a good-sized onion, minced or grated, make a depression in the centre of the chopped fish and add water, bread crumbs, salt and pepper to taste, one-fourth cup of sugar, two egg whites and two tablespoons of melted butter. Chop until very smooth and form into cakes containing a generous tablespoonful each. Put the bones and skins into a saucepan with an onion sliced and a tablespoon of butter and add the fish cakes. Cover with water and simmer for one and a quarter hours. Then remove the cakes and strain off the gravy into the two egg yolks which have been slightly beaten together with one teaspoon of sugar; stir over the heat until thickened, but do not boil it. Pour over fish cakes and serve either hot or cold. The butter and sugar may be omitted if so desired.