Preparing the Broth: In a large pot, bring the fish broth to a gentle simmer over medium heat.
Adding Fish Sauce: Stir in the fish sauce to add depth and umami to the broth, similar to what miso paste would provide in traditional miso soup.
Seasoning: Season the broth with salt, adjusting according to your taste.
Cooking the Fish: Add the thinly sliced fatty fish to the simmering broth. The thin slices will cook quickly, requiring just a minute or two to become tender and cooked through.
Optional Seaweed: If using, add seaweed to the pot in the last minute of cooking. Ensure the seaweed is a type that's acceptable within your dietary variation.
Serving: Carefully ladle the soup into bowls, making sure to include slices of fish in each serving.