Rinse salted mustard greens under running water; Soak it in water for 10 minutes; Remove it; Squeeze and slice it.
Rinse, drain, peel and slice ginger.
Rinse and drain a carrot; Trim a bit off each base; Peel it with a Y-shaped peeler; Rinse it under running water; Drain and julienne it.
Rinse celery under running water; Drain it; Trim a bit off the bottom; Pull off the outer leaves and tough stalks; Discard any wilted leaves; Cut the remaining into sections; Rinse them under running water again; Drain them.
Hold Chinese perch firmly by the head and scrape the scales from the tail towards the gills with a fish scaler; Rinse the fish under running water; Insert the knife into the anus near the tail; Draw the knife toward the head, splitting the fish to the base of the gills; Spread the abdominal cavity with fingers; Reach in and pull out the entrails; Rinse the cavity under running water; Cut off the head with the knife; Rinse the fish under running water again; Pat-dry the fish with kitchen paper; Cut it into pieces.
Fill a pot with 2.5L water; Bring to the boil over a high heat; Turn off the heat.
Add 2 tbsp oil to a anodized aluminum saucepan and swirl the pan; Add the ginger; Stir it over a high heat till fragrant; Turn to a low heat; Arrange the fish steaks into the pan; Turn to a high heat; Pan-fry the fish steaks over a high heat till both sides are lightly brown; Pour the boiling water from the step 6 into the pan; Turn to a low heat; Cook over a low heat for 8 minutes; Add all the remaining ingredients; Bring to the boil over a high heat again; Cook over a high heat for 15 minutes; Stir seasonings into the soup; Turn off the heat; Arrange the soup into serving bowls; Ready to serve.