Wash tuna belly and pat dry.
In a bowl, combine lemon juice, garlic, chili sauce, oyster sauce, sesame oil, salt, and pepper. Stir until well-blended.
Add tuna and marinate in the refrigerator for at least 4 hours or overnight, turning fish occasionally in the marinade.
Drain tuna, reserving the liquid. In a sauce pot, bring marinade to a boil for about 4 to 5 minutes or until reduced.
On lightly-greased grates over hot coals, arrange tuna in a single layer and grill.
When it starts to lose its pink, begin basting with the marinade. Continue to cook for about 3 to 5 minutes on each side, basting regularly, or until fish flakes easily with a fork and slightly caramelized. Remove from heat and serve hot.