Drain the water off the mackerel fillet well. Sprinkle them with salt and pepper.
Heat the oil in a pan over medium heat, stir fry napa cabbage stem-first and carrot. Add the Broth ingredients, bring to a simmer.
Add the mackerel and simmer for about 5 mins with a lid.
Add the napa cabbage leaves, chinese chives and keep simmering for about 2~3 mins. Add grated ginger, stir once.
Add dissolved mixture of potato starch and water, and stir well to thicken the broth with them.