Put the fish on in boiling water enough to cover, and which contains one tablespoonful of salt. Cook gently twenty minutes; then lift out of the water, but let it remain on the tray. Now carefully remove all the skin and the head; then turn the fish over into the dish in which it is to be served (it should be stone china), and scrape off the skin from the other side.
Pick out all the small bones. You will find them the whole length of the back, and a few in the lower part of the fish, near the tail. They are in rows like pins in a paper, and if you start all right it will take but a few minutes to remove them. Then take out the back-bone, starting at the head and working gently down toward the tail. Great care must be taken, that the fish may keep its shape.
Cover with the cream, and bake about ten minutes, just to brown it a little. Garnish with parsley or little puff-paste cakes; or, you can cover it with the whites of three eggs, beaten to a stiff froth, and then slightly brown.
Put the milk, parsley and onion on to boil, reserving half a cupful of milk to mix with the flour. When it boils, stir in the flour, which has been mixed smoothly with the cold milk. Cook eight minutes. Season highly with salt and pepper, add the butter, strain on the fish, and proceed as directed.