Rinse salted mustard greens under running water; Soak it in water for 10 minutes; Remove it; Squeeze and chop it finely.
Peel and chop garlic finely.
Rinse and drain spring onion; Cut off any roots; Chop it finely.
Add 2 tbsp oil and the garlic into a wok; Stir-fry it over a medium till fragrant; Add the preserved mustard greens, the spring onion and seasoning (1); Stir-fry it over a medium heat for 10 minutes; Turn off the heat.
Set an egg separator over a basic glass bowl; Break eggs into the egg separator, draining the egg white into the bowl.
Rinse fish steaks; Pat-dry them with kitchen paper; Remove any bones with a pair of tweezers, and discard any skin; Slice them.
Add seasoning (2) and the egg white into a big mixing bowl; Mix; Add the fish slices; Marinate them in the seasoning mixture for 30 minutes.
Rinse, drain, peel and chop ginger finely.
Add 4 tbsp oil into a nonstick wok; Heat it over a high heat; Turn to a low heat; Arrange the fish slices into the wok; Pan-fry them over a low heat for 4 minutes; Set the fish slices aside; Turn off the heat; Pat-dry the fish slices with kitchen paper gently.
Set the oil aside, leaving 1 tbsp oil into the wok; Add the ginger; Stir-fry it over a high heat till fragrant; Return the fish slice to the wok; Add the preserved mustard greens mixture; Stir-fry them over a high heat for 2 minute; Turn off the heat; Arrange them in a serving plate; Ready to serve.