Place the fish stock in a large, wide stockpot and bring to a simmer over medium heat. Reduce the heat to medium low and maintain a low simmer.
Place the onion, carrot, and parsnip in a food processor fitted with a blade attachment. Process until the vegetables are very finely chopped, stopping to scrape down the sides of the bowl as needed, about 1 minute total.
Transfer to a large bowl and set aside.
Cut the fish into 1-1/2-inch pieces.
Place half of it in the food processor and process until a ball has formed, about 30 seconds.
Transfer the ball to the bowl with the vegetables and repeat with the remaining fish.
Sprinkle the fish mixture and vegetables with the matzo meal.
Add the eggs, lemon zest, lemon juice, and measured salt and pepper. Using clean hands, mix until combined (don’t squeeze or overwork). To taste for seasoning, form a small patty and poach it in the fish stock until firm and cooked through. Taste the patty and add more salt and pepper to the fish mixture as needed. Repeat the seasoning test as needed.Using wet hands and a 1/4-cup measure, form the fish mixture into 3-inch-long ovals.
Place on a baking sheet.Gently place the ovals in the simmering stock. Cover with a tightfitting lid and simmer until firm and cooked through, about 10 minutes. Using a slotted spoon, carefully remove the ovals from the stock and transfer to a clean baking sheet to cool.
Pour the stock through a fine-mesh strainer set over a large heatproof bowl, discarding the solids. Allow the stock to cool to room temperature.Meanwhile, place the cooled gefilte fish in a large container with a tightfitting lid and refrigerate. When the stock is cool, pour it into the container with the gefilte fish, making sure they are submerged. Cover and refrigerate at least 1 hour and up to overnight.To serve, use a slotted spoon to transfer the gefilte fish from the stock to a serving platter.
Serve with beet horseradish or dill-horseradish mayonnaise, if using.
