Gefilte Fish

Cut fillets into 2-inch pieces. Attach a metal food grinder attachment with coarse grinding plate (8-millimeter) to a stand mixer. Position mixer bowl under attachment. Set mixer speed to medium-low, and grind fish. (Alternatively, pulse fish in a food processor, working in small batches, until finely chopped, about 8 pulses.) Cover ground fish in mixer bowl, and refrigerate while making the stock.

Place reserved whitefish head, bones, skin, and trimmings in a large stockpot. Add water, 2 of the quartered onions, half of the carrots, ½ cup of the parsley leaves and all of the stems, bay leaves, and peppercorns. Bring to a gentle boil over medium. Reduce heat to low and simmer until stock is reduced by half, skimming surface occasionally to remove and discard impurities, about 1 hour. Place a fine mesh strainer over a large heatproof bowl. Carefully pour fish stock through strainer into bowl; discard solids. Transfer stock to a 12-inch straight-sided skillet; season to taste with salt.

While stock is simmering, place 4 of the carrot pieces, remaining 1 quartered onion, and remaining 1/2 cup parsley leaves in a food processor. Pulse until finely chopped, 8 to 10 pulses, stopping to scrape down sides as needed. Add pulsed onion mixture, eggs, matzo meal, sliced chives, chopped dill, and salt to the ground fish; stir together using your hands until mixture comes together and is well combined. Divide mixture evenly into 12 (4-ounce) balls; shape into oval patties about 4 inches long and 11/2 to 2 inches thick. Place patties on a plate, and loosely cover with plastic wrap. Refrigerate until ready to use.

Add remaining 4 carrot pieces to strained fish stock in skillet. Bring to a simmer over low and adjust heat to maintain a temperature of 180°F. Working in about 3 batches, add fish patties to simmering stock and simmer, gently turning patties occasionally to ensure they don't stick to bottom of skillet, until patties are cooked through and a thermometer inserted into thickest portion of patties registers 140°F, about 15 minutes per batch. Transfer cooked patties to a plate.

Remove carrot pieces from skillet; let stand until cool enough to handle, about 3 minutes. Thinly slice carrot pieces crosswise; add to plate with patties. Cover patties and carrots loosely with plastic wrap, and chill until cold, at least 4 hours or up to 12 hours.

Top each patty with a few slices of carrot, and garnish with fresh dill fronds and chives. Serve gefilte fish cold with beet horseradish, if desired.