Put Japanese eel in a deep container and rouse it with salt; Leave it for up to 2 hours to kill it and to remove much of its slime (recommend not to bury it completely in salt); Rinse it thoroughly in cold water and scrape it to remove final traces of its slime; Sprinkle salt on it; Make a cut from its anus about halfway down to the neck end with a filleting knife; Pull away any innards from the rib cage; Scrub the gut cavity and rinse it out carefully to remove all traces of blood from the backbone and throat; Rinse it under running water for 6 minutes; Cut it into 8 pieces.
Rinse salted mustard greens under running water; Soak it in water for 10 minutes; Remove it; Squeeze and chop it.
Rinse and drain Chinese plums; Bisect them lengthwise; Core and chop them.
Rinse and drain red chili peppers; Cut off any stems; Scrape any seeds out; Chop them.
Peel and chop garlic.
Rinse, drain, peel and chop ginger.
Add seasonings into a big mixing bowl; Mix; Add the salted mustard greens, the garlic, the Chinese plums, the ginger and the red chili peppers; Mix.
Arrange the eel into a big mixing bowl; Pour an half of the mixture from the step 7 over the eel; Toss the eel in the seasoning mixture; Leave it for 30 minutes.
Rinse salted mustard green leaves under running water; Soak them in water for 10 minutes; Remove them; Pat-dry them with kitchen paper.
Fill a pan with water; Bring to the boil over a high heat; Stir 1 tsp caster sugar into the water; Stir it over a high heat till it dissolves; Turn to a low heat; Add the salted mustard green leaves; Blanch them over a low heat for 15 minutes; Turn off the heat; Immediately remove them; Leave them to cool.
Lay a piece of mustard green leaf on a plate; Place a piece of the eel in the middle of the leaf; Spoon some mixture from the step 7 over the eel; Fold the eel with the mixture in the leaf (repeat the step for each piece of eel)
Fill a deep wok with water; Stack a bamboo steamer in the wok; Bring to the boil over a high heat.
Arrange the wrapped on a steaming plate; Arrange the steaming plate in the bamboo steamer; Steam over a high heat for 16 minutes; Turn off the heat; Ready to serve.