Make four deep incisions (to the bone) into each side of the fish. Insert the pieces of ginger into the incisions and refrigerate fish until ready to cook. In a small sauce pan, combine the sake, soy sauce, lemon juice and about 1/8 tsp corn starch and bring to a boil. Then remove from heat and stir in the scallions. Preheat the grill to medium high. Generously sprinkle salt an pepper over the fish and spray (or rub) the outside with olive oil. Put the fish in a large fish basket, close tightly, and place the fish basked on the grill. Grill for 6 minutes on one side (with grill closed over basket). Then flip and grill for 6 minutes on the other side. Check for doneness. Fish should be firm but still somewhat soft. When done, remove fish from grill, place in a large bowl, and cover with the sauce from the sauce pan. Sprinkle with optional cilantro, scallions, and extra lemon juice.