Rinse dried shiitake mushrooms under running water; Soak them in water for 15 minutes; Remove them; Fill a bowl with 250ml water; Put the dried shiitake mushrooms into the bowl of the water; Leave it for 3 hours; Remove and drain them; Discard the stems with a pair of kitchen scissors; Cut the remaining into halves with a vegetable knife.
Put fish paste in a big mixing bowl; Add seasoning (1); Stir it clockwise until sticky (about 10 minutes); Cover the bowl with cling film wrap; Chill it in the fridge for 1 hour; Take the bowl out of the fridge; Discard the cling film wrap; Stir it clockwise again for 1 minute; Put a portion of the fish mixture on a hand and roll it into a ball (repeat this process till the fish mixture is used up).
Rinse, drain, peel and slice ginger.
Rinse mustard greens under running water thoroughly; Drain them; Trim a bit off the bottoms; Discard any wilted outer leaves; Cut the remaining into pieces.
Add 2 tsp oil to a nonstick casserole; Add the ginger; Stir-fry over a high heat till fragrant; Add the shiitake mushrooms together with their soaking water and the mustard greens; Pour 110ml water into the casserole; Bring to the boil over a high heat and simmer it for 10 minutes; Add the fish balls; Bring to the boil over a high heat again; Turn to a low heat; Stir seasoning (2) into the mixture; Cook over a low heat for 6 minutes; Turn off the heat; Arrange it in a serving bowls; Ready to serve.