Combine the soy sauce, lime juice, ground black pepper, and red chili in a medium-sized bowl. Mix thoroughly.

Add the minced garlic and brown sugar to the mixture. Stir well until the sugar has completely dissolved and the marinade is smooth.

Place the tuna jaw into a resealable plastic bag. Carefully pour the marinade into the bag, ensuring the tuna is fully coated. Remove any excess air from the bag before sealing it. Transfer the sealed bag to the refrigerator and allow the tuna to marinate overnight for maximum flavor absorption.

Prepare the basting sauce by mixing the soy sauce and white sugar in a small bowl. Stir until the sugar is fully dissolved. Set the sauce aside.

Preheat your grill to medium-high heat. Place the marinated tuna jaw on the cooking grate. Generously brush the top of the fish with the prepared basting sauce. Grill for 3 to 5 minutes.

Carefully turn the tuna over to cook the other side. Baste the top and sides with the sauce. Continue grilling for an additional 5 minutes.

Flip the tuna jaw again to ensure even cooking. Allow this side to grill for 5 to 7 minutes, basting as needed.

For the final grilling, flip the tuna jaw one last time and cook the opposite side for 5 more minutes, ensuring the fish is fully cooked and slightly charred.

Once done, transfer the grilled tuna jaw to a serving plate. Serve it with a zesty dipping sauce made by combining soy sauce, lime juice, and chopped chili.

Enjoy this savory and flavorful dish with your loved ones!