For the dashi: Rinse the kombu and place in 2 cups water over high heat until boiling. Add bonito flakes, turn off the heat and let rest for 15 minutes. Strain through cheesecloth or a clean dishtowel. Set aside.
Place the hijiki in 1 cup of cold water and let rest to reconstitute, 5 minutes. Meanwhile, slice and julienne the white part of the scallion in a 1 ½ -inch length. Thinly slice and julienne the darker green scallion tops for garnish. Peel, thinly slice, and julienne the ginger. In a heatproof shallow dish, place half of the ginger and scallions on the bottom. Then, layer the fish on top adding black pepper as the fish is layered. Add the remaining ginger and scallions over the fish.
Finish the dish by pouring soy sauce, mirin, and oil evenly over the top, followed by zested lemon. Strain the hijiki and rinse under cold water, then layer it down the center of the dish. Finish the dashi by straining the kombu/bonito flake mixture through cheesecloth.
Pour the dashi into a large pan big enough to accommodate the fish dish and heat to boiling. Place a small rack or scrunched up piece of tin foil on the bottom to elevate the fish as it cooks. Place the fish dish on top of the foil, over the boiling dashi. Cover and steam for 5-7 minutes.
The fish is cooked when it’s opaque throughout and a cake tester is easily inserted into the thickest portion of the fish. Garnish with the scallion greens and serve.