Cut fish into 2-inch slices. Carefully remove flesh and bones without
breaking skins. Salt the skins and bones and place in a covered bowl
in refrigerator while preparing filling.
Put the flesh and 1 onion through a food chopper, then place in a wooden
chopping bowl and cut until smooth. Add eggs, matzo meal or
breadcrumbs, 1 teaspoon salt, pepper to taste, and enough cold water
to make a light soft mixture. Blend thoroughly.
Wet hands with cold water. Form oval cakes of mixture and fit them into skins. Place head bones and any other bones in the bottom of a deep heavy kettle. Add oval cakes of fish in skins on top. Add remaining onion, celery, carrot, and cold water to cover. Cover kettle and bring to a quick boil. Remove cover. Reduce heat and cook at avery slow boil 1 1/2 to 2 hours. The liquid should be reduced by half. Allow to cool slightly before removing fish to a platter carefully so as to retain shape.
Serve warm or thoroughly chilled, garnished with sliced, cooked carrot and chopped parsley. If served cold, use the jellied sauce as a garnish. Serve with prepared horseradish.