Marinade:.
Combine garlic, cilantro, fish sauce, soy sauce, sugar, salt, pepper, and oil in a food processor or blender and grind to a smooth paste.
Transfer to a large zip-lock bag and add fish fillets.
Turn gently to season well.
Set aside for 30 minutes, or refrigerate up to 1 day.
Sauce: Combine sauce ingredients in a small bowl, stirring to dissolve sugar.
Sprinle with chili and cilantro and set aside.
Coat a grill rack with cooking spray and preheat, or lightly oil a shallow baking pan and preheat oven to 425 degrees.
Remove fish from marinade and cook until done, turing once, about 5 minutes per side.
Arrange on serving platter along with a small bowl of sauce, if using, to spoon over the fish.
Serve hot or warm.