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 1/4 cup of the finished rice and flatten it onto the leaf, within 1-inch of the sides, and 2 inches 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.