Rinse fillets and pat dry with paper towels. In a medium bowl mix together the soy sauce, mirin, sugar and fresh ginger. Place fillets into the marinade and let stand for at least 20 minutes.
Preheat your grill or an outdoor barbecue for high heat. Cook 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.