Combine all liquid and heat. Add rice and cook, covered, until tender. Set aside to cool. Fill a small sauce pan with water, add 1 teaspoon salt, and bring to a boil. Blanch the napa leaves in the water for 2 minutes. Refresh in some cold water. Drain. Season the cod fillets with salt and pepper. Brush 1 teaspoon of oyster sauce on each piece. Lay out the napa leaves inside up, stem end toward you. Take quarter cup of the finished rice and flatten it onto the leaf, within 3 cm of the sides, and 5 cm of the top and bottom. Place the cod in the middle of the rice. Sprinkle it with a pinch of the chopped ginger. Fold in the sides of the leaves to the center. Fold the stem over the top of the fish and continue to roll, making a bundle. Continue with the rest of the fish. Place seam side down on the rack of your steamer, cover and steam for 15 minutes. Serve with mustard vinaigrette