Rinse fillets, and pat dry with paper towels. In a medium bowl, mix together the soya sauce, mirin, sugar and fresh ginger. Place fillets into the marinade, and let stand for at least 20 minutes.
Preheat your ovens broiler, or an outdoor grill for high heat. Broil the fillets, basting occasionally, until the fish flakes easily with a fork, about 5 to 8 minutes. Serve with a lemon slice or long white radish slice as a garnish.
The nutrition data for this recipe includes information for the full amount of the marinade ingredients. Depending on marinating time, ingredients, cooking method, etc., the actual amount of the marinade consumed will vary.