Place tuna in a shallow baking dish or bowl. Combine remaining ingredients well, then pour over tuna. Cover fish and place in refrigerate to marinate for 30 minutes, turning once. Grill about 3-4 minutes on each side over moderately hot coals. Serve immediately with Wasabi Sauce. Makes 4 generous servings.
1 cup low fat sour cream or mayonnaise
2 teaspoons wasabi
1/2 teaspoon powdered ginger
1 teaspoon soy sauce
Combine all ingredients in a small bowl. Sauce should be served at room temperature. Makes one cup of sauce.