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1
Combine the fish bones, skin and heads in a 10-quart or larger stock pot.
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2
Add half of the onions and carrots and all the celery.
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3
Place the fish fillets in a food processor and pulse until coarsely ground (you may have to do this in batches).
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4
Set aside.
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5
Put the remaining onions and carrots in the food processor and chop medium fine.
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6
Mix with the ground fish by hand.
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7
Soak the matzohs in the seltzer for 5 minutes, then squeeze dry and crumble.
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8
Add the matzohs, whole eggs, egg yolks and half the salt and all the pepper to the ground fish mixture.
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9
Lightly shape the fish mixture into balls 2 to 2 1/2 inches in diameter.
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10
If the balls will not hold their shape or if mixture feels too sticky, add matzo meal until it is the consistency of meatballs.
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11
Place the balls in the pot on top of the bones and vegetables.
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12
They can be in a double layer.
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13
Add cold water to cover the fish balls almost completely.
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14
Add lemon juice and remaining salt.
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15
Bring to a boil; reduce heat and simmer uncovered for 1 1/2 to 2 hours, or until the balls are solid and the liquid has been reduced by half.
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16
Frequently baste any uncovered balls as they cook.
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17
Remove fish balls with slotted spoon.
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18
Place on a platter and wrap with plastic wrap and refrigerate.
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19
Let broth simmer to reduce again by half.
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20
Cool; strain and pour a light layer over the fish balls.
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21
Chill.
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22
Broth will gel.
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23
Serve with fresh horseradish (recipe below).
