Ruth Messinger's Gefilte Fish

10 pounds whole fish (a combination of whitefish, pike and carp — but no more than 1/3 carp), cut into fillets (reserve bones, skin and heads)
8 medium onions, quartered
8 medium carrots, roughly chopped
6 stalks of celery with leaves, roughly chopped
3 to 4 whole matzohs
2 cups plain seltzer
6 whole eggs
3 large egg yolks
2 tablespoons salt or to taste
1 teaspoon pepper
1 cup matzoh meal as needed
¼ cup lemon juice

Nutritional Information
- Nutritional analysis per serving (40 servings)
  152 calories; 2 grams fat; 0 grams saturated fat; 0 grams trans fat; 0 grams monounsaturated fat; 0 grams polyunsaturated fat; 7 grams carbohydrates; 0 grams dietary fiber; 1 gram sugars; 24 grams protein; 91 milligrams cholesterol; 430 milligrams sodium

40 fish balls

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