For peanut sauce, in a blender container or food processor bowl combine the 1/2 cup peanuts, the 1/4 cup water, the green onion pieces, sesame oil, soy sauce, rice vinegar, sugar, ginger, and garlic. Cover and blend or process until almost smooth; pour into a small saucepan. Stir in 2 to 3 tablespoons water; set aside.
In a small bowl combine teriyaki sauce and the 1 tablespoon water. Rinse fish; pat dry with paper towels. Brush both sides of the fish with the teriyaki mixture. Place fish on the greased rack of the grill directly over medium heat. Grill for 4 to 6 minutes per 1/2-inch thickness of fish or until fish flakes easily when tested with a fork, turning once halfway through grilling.
In a saucepan slowly warm peanut sauce over medium-low heat. (Sauce will thicken slightly as it is heated. If needed, stir in additional water until desired consistency.) Spoon sauce over tuna. If desired, garnish with chopped peanuts and sliced green onion. Makes 4 servings.