Heat the oil in a large pan over medium heat. Add the onions and sauté until softened but not brown, about five minutes. Set aside to cool.

Combine the ground fish, one-third of the cooked onion (about 3/4 cup/90 grams), the eggs, two teaspoons salt, one teaspoon pepper, one tablespoon of the sugar, club soda, and 1/4 cup (25 grams) of the bread crumbs. Add another one to two tablespoons of bread crumbs if the mixture seems too loose. Set 1/2 cup of mixture aside, or enough to fill the head of the fish.

Bring eight to 10 cups (2 to 2.4 liters) water to a boil.

Place the remaining cooked onions in the bottom of a large Dutch oven, wide stockpot, or fish-poaching pan. Cut a 12 x 15-inch (30.5 x 38-centimeter) piece of parchment paper or cheesecloth and lay it on a flat surface. Place the fish mixture in the center and pat into a 10 x five-inch (25 x 12-centimeter) rectangle about one inch (2.5 centimeters) tall. Take the sides of the parchment or cheesecloth and fold them around the rectangle, creating a package to hold the shape. Secure the folds in place with a toothpick if you like.

Gently place the wrapped fish on top of the onions in the pot. Pour boiling water over the dish to cover (you will not need all of it; keep the remaining water at a boil to be used in step six). Add the remaining tablespoon of sugar, one tablespoon salt and one teaspoon pepper to the water. Bring to a gentle simmer and cook for three and a half to four hours, uncovered until the fish is firm.

Meanwhile, rinse the fish head with cold water until it runs clear. Fill the fish head with the reserved fish mixture from step two. Wrap the head and the tail together in parchment paper and place in a pot. Cover with boiling water and simmer for one to one and a half hours, until fish mixture is firm.

When all the fish is done cooking, gently remove it from the pot and remove the parchment. Let it cool, then refrigerate for a few hours or overnight.

To serve, place the cooked fish rectangle in the center of a large serving platter. Set the cooked head at one end and the tail at the other.

Garnish with carrots and parsley, and serve with horseradish on the side. It is best served cold or at room temperature.