Rub the tuna steaks with 4 tablespoons sugar. Allow to sit 30 minutes in the refrigerator.
In a frypan over medium heat, toast the sesame seeds 5 minutes or until lightly browned.
In a shallow bowl mix the remaining sugar, toasted sesame seeds, spring onion, garlic, ginger, soy sauce, sesame oil, salt and pepper. Place the tuna steaks in the mixture and marinate 2 hours in the refrigerator.
Preheat the grill.
Discard marinade and place the tuna steaks on a baking tray. Grill to desired doneness in the preheated oven.