First, clean the mackerel thoroughly and rinse it under cold water. Dry it with paper towels to remove excess moisture. This step is crucial to ensure the fish is ready for filleting.
Using a very sharp knife, filet the mackerel along the bones from the tail up towards the head. Take your time and repeat on the other side. The goal is to get as much meat off as possible while discarding the bones and skin.
Put the fish meat into a bowl and start scraping the meat off the skin of each filet with a spoon, from the tail upwards until all the meat is removed. Discard the skin once you're finished.
Next, scrape the meat off the fish bones as well, ensuring no meat is wasted. Gather all the fish meat on a chopping board.
Using a Chinese cleaver (or any sharp knife), chop the fish meat for about 10 minutes. It will become slightly sticky as you chop, and this texture is essential for shaping the balls later.
After chopping, add the salt and white pepper. Continue to chop for another 10 minutes; the mixture should transform into a paste-like consistency.
Wet your hands with water, take about a tablespoon of the fish paste, and shape it into small balls. Repeat until all the mixture is shaped into fish balls.
In a 5-quart soup pot, pour in the chicken broth and water, adding the chopped fish bones. Bring the mixture to a boil over high heat, then reduce the heat and let it simmer for about 20 minutes. This will allow the broth to become rich and flavorful.
Remove the fish bones from the pot, then increase the heat back to high. Gently drop the fish balls into the boiling soup.
Add the Chinese seaweed, soy sauce, sesame oil, and season with additional salt and white pepper to taste. Once the fish balls float to the top, it's time to turn off the heat.
Transfer the soup along with the fish balls into serving bowls and garnish with shredded lettuce and scallion rings. Serve immediately and enjoy!