On a cutting board, press the rice into a layer about three-quarters of an inch thick. Cover the rice with the fish.
With a sharp knife, cut the pad of fish-topped rice into pieces about three inches long and one inch wide.
In a very clean bowl, beat the egg whites with the cream of tartar until it begins to peak. Add the wasabi powder and continue whipping till it gets stiff. Add the diluted soy sauce and stir in with a rubber spatula. Leave faint soy-sauce streaks in the egg-white foam.
Place the pieces of sushi, about two inches apart, on a ceramic or glass baking dish, or on one of those insulated baking sheets.
With a spoon, cover each piece of sushi completely with the egg white mixture, forming oblong mounds.
Bake the foam-covered sushi in the 400-degree oven until the foam browns lightly--about two minutes.
Using a small metal spatula, place two pieces of sushi on each plate, and garnish with green onions.