Combine soy sauce, lime, ground black pepper, and chili in a medium bowl. Stir.
Add garlic and sugar. Stir thoroughly until the sugar dissolves.
Place the tuna in a re-sealable plastic bag. Pour the soy sauce marinade along with the tuna. Let the air out of the bag and then seal it. Place inside the refrigerator and let it marinate overnight.
Prepare the basting sauce (use the basting sauce ingredients above) by combining soy sauce and white sugar in a bowl. Stir well. Set aside.
Heat the grill. Start to grill the Tuna by arranging it over the cooking grate. Brush a generous amount of basting sauce on the top part of the fish. Grill for 3 to 5 minutes.
Turn the tuna over to cook the opposite side. Baste the top and sides. Continue to grill for another 5 minutes.
Gently flip the fish jaw to grill the other side one more time. Cook it for 5 to 7 minutes.
Once done, flip it back for the last time to cook the opposite side. Cook it for 5 minutes. Transfer to serving plate. Serve with a dipping sauce composed of soy sauce, lime, and chili.
Serve. Share and enjoy!