Mince 3 scallions and set aside and also slice 3 scallions at a thin diagonal and set aside.
Use a vegetable peeler and peel the skin off the ginger, then using a grater, finely grate the ginger into a bowl, then set aside.
Remove 6 whole cabbage leaves and set aside and then thinly slice the rest of the cabbage and set in a bowl.
In a bowl, stir together chopped cilantro if using, minced scallions, ginger, 2 Tbsp soy sauce and 1 Tbsp oil.
Then add the cod fillets to the mixture, coat well, and let stand for 10 minutes at room temperature.
Place a steamer basket into a large pot and add enough water to the pot without reaching the basket, then cover the pot and turn the heat to medium-high.
Remove the basket and place 4 cabbage leaves onto the steamer basket while flat, covering the cod with the remaining 2 cabbage leaves.
When the water is boiling, remove the pot from the heat and replace the steamer basket with cod and cabbage into the pot, then cover and let steam for 8-12 minutes.
In a small bowl, whisk the remaining 4 Tbsp soy sauce, 2 Tbsp rice vinegar, 2 tsp sugar, salt and pepper, making sure to mix this sauce well as it will be then used to season the cabbage.
Transfer 3 Tbsp to the bowl with the sliced cabbage and toss.
Next, arrange the cabbage shreds onto a serving platter while reserving the remaining dressing for topping the cooked cod.
When the ginger-scallion cod is ready, remove the cover and cabbage leaves covering the fish, and placing the fish to a serving platter, laying it on top of the prepared cabbage shreds.
Sprinkle the fish with the sliced scallions and serrano if using.
In a small bowl, warm the 2 Tbsp oil until just before smoking and carefully pour the oil over the fillets, as it will sizzle.
Finally, sprinkle with the cilantro leaves and serve with reserved dressing over the fish.