To prepare the fish: In the workbowl of a small food processor or a blender, combine the garlic, cilantro roots, fish sauce, soy sauce, sugar, salt, pepper, and oil. Grind to a fairly smooth paste, stopping now and then to scrape down the sides and adding a little water as needed to bring the ingredients together. Scrape the cilantro-garlic paste into a medium bowl, add the fish fillets, and toss to coat everything well. Set aside for 20 to 30 minutes; longer is fine, covered and refrigerated, up to 1 day.
To make the sauce: Combine the fish sauce, lime juice, sugar, and garlic in a small bowl. Stir well until the sugar dissolves, and then sprinkle with the chilies and cilantro. Set aside until serving time.
Prepare a very hot fire in a charcoal grill, or heat a gas grill or oven to 425 degrees F. To cook the fish, place it on a lightly oiled grill rack, or in a shallow baking pan in the oven Cook until handsomely browned and done to your liking, carefully turning once, about 5 minutes on each side, or longer for thicker fillets. Serve hot or warm, with the bowl of sauce on the side.
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