Prepare the marinade by combine soy sauce, calamansi, ground black pepper, and sugar in a bowl. Stir until well blended.
Place the tuna in a Ziploc bag. Pour the soy sauce marinade.Let the air out of the bag. Seal. Marinate inside the fridge for at least 3 hours.
Heat-up the grill. Arrange the tuna belly on the grate of the grill. Brush a generous amount marinade mixture on the tuna. Grill for 3 to 5 minutes.
Turn the tuna belly over and grill the opposite side. Baste the top and sides. Continue to grill for another 5 minutes. Continue performing the process until the tuna is completely cooked.
Blanch or stir fry your vegetables. Season to taste and serve your grilled Tuna Belly on top of your vegetables
Serve with a dipping sauce composed of soy sauce, calamansi, and chili pepper. Share and enjoy!