Chill an onion in the fridge for 30 minutes before any peeling and cutting.
Rinse, drain and peel ginger; Grate it with a microplane grater over a bowl.
Put a strainer over a bowl; Line the strainer with a piece of cheesecloth, draping the corners over the sides; Spoon the grated ginger into the center of the cheesecloth; Gather up the 4 corners and twist to squeeze the grated ginger for juice for later use.
Add seasoning (1) and the ginger juice into a mixing bowl; Mix.
Rinse fish steaks; Pat-dry them with kitchen paper; Marinate them with seasoning (1) for 30 minutes.
Rinse and drain coriander; Slice off any roots and stems.
Core pickled plum; Chop it.
Take the onion out of the fridge; Slice off the stem of an onion; Peel off the outer layers of its skin; Cut the onion in halves from its stem end to root; Julienne it.
Rinse and drain tomatoes; Tweeze off any stems; Cut the tomatoes into quarters.
Add 4 tbsp cornstarch into a flat plate; Evenly dust the fish steaks with the cornstarch.
Add 2 tbsp oil to a nonstick casserole; Heat it over a high heat; Turn to a medium heat; Arrange the fish steaks in the casserole; Pan-fry them over a medium heat till both sides are slightly brown; Set the fish steaks aside; Turn to a low heat; Add the onion; Stir-fry it over a low heat till fragrant; Add seasoning (2); Stir it over a low heat till the muscovado completely dissolves; Add the tomatoes and the pickled plum; Bring to the boil over a high heat; Turn to a medium heat; Cook over a medium heat for 5 minutes; Turn to a low heat; Return the fish steaks to the casserole; Cover the casserole with its lid; Cook over a low heat for 6 minutes; Turn off the heat; Arrange it in a serving plate; Garnish it with the coriander; Ready to serve.