2 cups soy sauce
1/4 water
4 tablespoons chopped fresh ginger
1/2 cup chopped scallions
2 garlic cloves, chopped
pinch of red pepper flakes
2 teaspoons ground black pepper
lemon juice
olive oil

Thoroughly combine all ingredients except for the olive oil. Marinate tuna fillets or steaks in the sauce for 3 hours, turning every hour. Brush tuna with olive oil and cook under broiler for 3-5 minutes per side, depending on the thickness of the steaks.  Steaks can also be grilled.