Place tuna steaks in a plastic. Set bag in a shallow pan. In a small bowl, combine sesame oil, soy sauce, green onion, ginger, rice vinegar and garlic. Pour mixture over tuna in bag and seal. Refrigerate for 30 minutes, turning bag occasionally to coat. Lightly coat the grill rack with nonstick cooking spray. Drain tuna and discard marinade. Grill fish for 10 ro 12 minutes or until fish flakes with a fork (about 145 degrees F), turning once. Makes 4 servings.