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1
Ask your fishmonger to fillet the fish, reserving the bones and heads for you. Also ask for an extra head (it will improve the broth).
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2
Coarsely grind the fish with 2 of the onions in a grinder or processor.
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3
Transfer the mixture to a wooden bowl and continue chopping with a mezzaluna (half-moon shaped chopper) as you work in the 2 tablespoons salt, 1 tbsp sugar, matzo meal, eggs, and enough water (about 1/4 cup) to produce a smooth light paste. If you don't have a mezzaluna, you can do this on a cutting board with a knife (**either way, make a well in the center of the mixture before adding the eggs and water).
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4
Put the fish heads and bones into a large wide pot along with the carrots, pepper, remaining 2 tbsp salt, 1 tbsp sugar, and one onion. Cover with plenty of water and bring to a boil.
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5
In a separate pot, bring to a boil 3 quarts of water.
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6
When the first pot comes to a boil, prepare the fish balls. Keeping your hands moist with cold water, form spheres the size of very large eggs (they will expand when cooked) and drop them carefully, one by one, into the pot with the fish bones. The water should be kept at a slow simmer as you continue to add fish balls.
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7
After all the balls have been added, simmer for 1.5 hours. Add additional water from the second pot as necessary, so that there is enough liquid to keep the balls afloat.
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8
Remove from heat and let the fish balls cool in their liquid.
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9
With a slotted spoon, remove the fish balls to a serving platter.
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10
Strain the cooking liquid. Pour it into a large wide jar and refrigerate until cold (overnight if possible). It should gel. If it doesn't, rewarm the liquid and dissolve 1 package of gelatin in it. Test by putting a teaspoon of the liquid on a plate and refrigerating; if it doesn't gel, add more gelatin (up to 3 packages) until it does. Refrigerate until chilled.
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11
Serve the fish cold with the fish aspic and chrain.
