1. Combine the ingredients for the marinade in a shallow mixing bowl and whisk together. Add the tuna steaks and marinate for 30 to 60 minutes, turning the steaks once or twice. Keep the bowl covered in the refrigerator.
2. Preheat the grill to high. With a slotted spatula, carefully lift the tuna steaks out of the marinade. Strain the marinade into a small saucepan and boil to a thick, syrupy glaze.
3. Lightly spray the fish steaks with non-stick cooking spray or sesame oil. Brush and oil the grill grate. Grill the fish until cooked to taste, about 4 to 6 minutes per side for medium. As each side is cooked, brush it with the glaze (the boiled marinade). Sprinkle the tuna with chopped scallion greens and sesame seeds and serve at once.Serving Size: 1 tuna steak