Grill garlic and chiles: Preheat charcoal grill according to grill directions to medium heat, about 30 minutes. Meanwhile, peel garlic; slice larger cloves in half. Skewer garlic and place on the grill. Remove chile stems, skewer sideways, and place on the grill. Grill garlic and chiles for 8–10 minutes, flipping after 4 minutes. Note: Alternatively, garlic and chiles can be charred in the oven. Preheat to broil. Place on a baking sheet 4–6 inches under the heat source for 6–8 minutes, flipping halfway through, until both sides are charred.
Cilantro: Place a cloth towel under the mortar to keep it stable. Trim half of the cilantro stems, roughly chop, and place in the mortar along with the salt. Use pestle to pound the stems and salt into a paste. Finely chop remaining cilantro bunch (stems and leaves); set aside.
Sauce: Garlic and chiles are done when they’re soft, cooked through, and lightly charred. Remove from skewers and place into the mortar. Pound into the cilantro-salt mixture to break the ingredients down to a smooth paste. Add sugar, lime juice, and fish sauce to the mortar; stir around to combine. Add 2 tablespoons chopped cilantro (if there’s any remaining, save for another recipe); transfer to a bowl and serve. Makes about ¾ cup. You can store it in the refrigerator in a sealed container for several days, but it tastes best immediately after it's made.
This is a "naam jim" (dipping sauce) meant to be served with the Salt-Crusted Whole Fish. It pairs well with other seafood dishes, too.