To prepare the stock: Place sliced onions, celery and carrots in the
bottom of a large soup pot, then add fish skin, bones and heads.
Season with salt, sugar, pepper and paprika, if desired. Add water
to cover and cook for 45 minutes at medium-high heat. Add more water
if necessary; do not allow the pot to overboil. When finished,
remove from burner and strain, discarding all but carrots; set aside
carrots and stock separately.
To prepare fish balls: Place the ground fish in a wooden chopping
bowl. Mix the fish varieties together thoroughly. Stir in puréed
onion, then stir in beaten eggs, salt, sugar and pepper. Slowly add
matzo meal and 1 cup water, combining thoroughly, and adding more
water if mixture is too heavy. Taste for seasoning and add more salt
and pepper, if necessary. Continue stirring the mixture until able
to stick a knife blade inserted into chopped fish comes out clean.
Form the fish paste into balls about the size of tennis balls. Place
them in a large metal pan without crowding atop the burner. Add hot
stock to cover the fish balls and cook on top of the stove at a
medium heat for about 30 to 45 minutes. Do not allow the liquid to
boil over. When done, fish balls should change from pink to white.
Add more stock as necessary, making sure the fish balls do not stick
to the bottom of the pan. Cover pan with aluminum foil and transfer
to a preheated 325-degree oven. Allow to bake for 1 1/2 hours, then
remove the aluminum foil, and bake until the tops of the fish balls
are lightly brown, about 30 minutes. Remove fish balls from the
oven, and allow to cool. Refrigerate until ready to serve. Garnish
with reserved sliced carrots.