Crush the lemongrass with the blade of a chef’s knife and remove the outer layer, finely slice a 4-5cm piece from the root end and set aside. Finely slice the ginger and place the ginger and lemongrass into the bowl of a mortar and pestle.
Pound the ginger and lemongrass to a fine paste and transfer from the mortar and pestle to a small mixing bowl. Add black vinegar, soy and sweet soy to the lemongrass and ginger and mix briefly.
Place the rockling in a zip lock bag and pour in the soy marinate. Seal the bag and place the bag onto a perforated tray and into a pre-heated oven steam setting 65 °C for 35 minutes.
Slice the corn in half and set aside with picked coriander sprigs.
Once the fish is cooked adjust the temperature of the oven to 100 °C steam setting. Remove the fish from the tray and place the corn on the tray and into the oven for 7 minutes.
Once the corn is cooked remove it from the oven and mix it with the picked coriander, salt and evoo.
Place the corn on a serving dish and open the bag and pour off some of the poaching liquid. Place the fish on top of the corn and pour over a little of the poaching liquid. Finish with a drizzle of evoo and serve.