Homemade Gefilte Fish

Separate dark-green leaves from leek; wrap thyme, parsley stems, bay leaves, and peppercorns in leaves and tie closed with kitchen twine; set aside. Slice remaining pale-green and white parts of leek in half lengthwise.

Fillet fish to yield 3 lb. meat, reserving bones. Discard pin bones. Cut bones into large pieces; reserve fillets (a fishmonger comes in handy here!).

Place bones in a large stockpot; pour in cold water to cover. Bring to a boil, drain immediately, and return bones to pot. Add cold water to cover and bring to a boil. Add onion, celery, carrot, wine, and pale-green and white parts of leek; reduce heat and bring to a simmer. Add reserved leek bundle to pot. Season lightly with salt and simmer until stock is pale golden and fragrant, 40–45 minutes. Remove from heat and remove aromatics and large fish bones with a slotted spoon; discard.

Strain stock through a fine-mesh sieve into a large straight-sided skillet; bring to a gentle simmer over medium-low and keep warm while you prepare the fish.

Do Ahead: Stock can be made 1 day ahead. Let cool; cover and chill.

Heat oil in a medium skillet over medium. Cook onion, parsnip, and leek, stirring often and reducing heat as needed to prevent vegetables from browning, until onion is translucent and vegetables are very soft, 10–12 minutes. Let cool.

Meanwhile, cut reserved fish fillets into 1" pieces. Working in 3 batches, pulse fish in a food processor until finely ground (pluck out any errant pin bones you find). Transfer to a large bowl.

Pulse eggs, salt, sugar, white pepper, nutmeg, cayenne, and cooled onion mixture in food processor until mixture is frothy and mostly smooth with only a few bits of onion still visible. Pour over fish and mix with your hands to incorporate. Scatter matzo meal over and mix in with your hands until homogenous (be careful not to overmix; otherwise, mixture will be dry and crumbly after it cooks).

If making quenelles for individual servings, hold 2 large spoons (they should be the same size) in each hand and scoop out about ⅓ cup fish mixture with 1 spoon. Holding spoons parallel to one another, place edge of empty spoon over fish mixture and, while rotating the empty spoon, scoop fish mixture out of first spoon. Repeat motion several times, passing fish mixture between spoons, to make a smooth football-shaped scoop (a quenelle) and place on a parchment-lined rimmed baking sheet. You should have 25–30. (Alternatively, you can use a 2½-oz. ice cream scoop to make uniform balls.)

Working in batches (about 10 per batch), gently lower quenelles into simmering stock with a slotted spoon and poach until cooked through, about 5 minutes. Using slotted spoon, transfer to another rimmed baking sheet and tent with foil to keep warm.

Add carrots and pearl onions to stock and simmer until tender, 10–12 minutes.

Divide carrots and pearl onions among bowls with slotted spoon and add 2–3 fish pieces to each bowl. Strain fish stock again through a cheesecloth-lined sieve, if desired, and divide among bowls. Top with dill and serve with lemon wedges and horseradish alongside.

If making large fishes for family-style serving, line 4 large plates with parchment. Transfer about one-quarter of fish mixture to 1 plate. Using wet hands or gloves, form into the shape of a fish (make sure it will fit in skillet with stock). Repeat with remaining fish mixture and plates.

Working with 1 fish at a time, carefully lower into simmering stock, using parchment paper to help you (if your skillet is large enough, add another fish). Poach, turning once if not fully submerged, until cooked through, 7–10 minutes. Using 2 long spatulas, transfer to a rimmed baking sheet and let cool.

To serve, transfer fish to a platter and slice ½" thick. Arrange dill alongside and serve with lemon wedges, capers, horseradish, and matzo.

Do Ahead: Gefilte fish can be poached 2 days ahead. Let fish and stock cool, then transfer to a wide, shallow baking dish, making sure fish is submerged. Cover tightly with foil and chill. Reheat fish and stock over medium-low.