Dry the fish well with a towel. Using a pestle or heavy pan, whack the thick end of the lemongrass several times to bruise it, which releases some of its oils. Insert the stalk through the fish’s belly so that the thin end comes out of the fish’s mouth. (You may have to use a knife to clear a path from belly to mouth.) Pull the thin end of the lemongrass firmly but gently so as much of the stalk sticks out of the mouth as possible, and tuck the thick end into the belly until it’s no longer visible.
Spread the salt on a large plate. Using a brush or your hands, coat the fish on one side with the beaten egg white in a thin layer from the head to the tip of the tail. Lay the fish, egg white-brushed side down, onto the bed of salt, gently pushing down on the fish so the salt adheres. Brush the other side with the egg white (there’s no need to use it all), flip the fish over, and use your hands to pat the salt on the fish to form a more or less even layer (thick enough to partially obscure the color of the skin, but not as thick as a European salt crust), enlisting some salt from the plate if necessary. Flip the fish once more and do the same on the other side. There will be a lot of salt left over. That’s fine. At this point, you can keep the fish in the fridge for 30 minutes to help the crust firm up or cook it right away.
ON THE GRILL (Highly recommended): Prepare a grill, preferably charcoal, to cook over high -- but not blazing hot -- heat. Put the fish directly on the grill grates and cook, without messing with it, for at least 6 minutes (otherwise, the crust could break). Then begin checking the underside, and when it’s a light golden brown with a few darker brown patches, use the spatula and a pair of tongs to carefully turn it over and cook until both sides are golden brown and the fish is just cooked through to the bone, about 16 to 20 minutes total (see Note, below).
IN THE OVEN: Preheat the oven broiler to high and position the oven rack as far as possible from the heat source. Line a baking sheet with aluminum foil and add a wire rack. Put the fish on the wire rack and broil, using a spatula and a pair of tongs to carefully turn it over once, but only after the crust has firmed up, until both sides are golden brown and the fish is just cooked through to the bone, 16 to 20 minutes (see Note, below).
Right before you serve it (perhaps at the table for dramatic effect), remove the crust (it’s too salty to eat and it’s very easy to do): Using the tip of a sharp knife,
make an L-shaped cut just through the salt crust, starting at the tail, cutting along the back toward the gills, then down toward the bottom of the fish. Peel back the skin in one piece, do the same on the other side of the fish, and discard both pieces or leave them on the plate as garnish. Serve with a bowl of the dipping sauce.
Note: Because the crust makes it difficult to inspect the flesh, the best way to check doneness is to use a thin, instant-read thermometer. The fish is done when the thermometer inserted into the flesh at the thickest part of the fish (behind the head at the fish’s back) registers 125° F.
I help chefs write cookbooks! I’ve co-authored several, including Pok Pok: Food and Stories from the Streets, Homes, and Roadside Restaurants of Thailand (Ten Speed) with Andy Ricker, A Girl and Her Pig (Ecco) with April Bloomfield, and Truly Mexican and Tacos, Tortas, and Tamales (Wiley) with Roberto Santibanez.