First, prepare the fish. Cut a few diagonal slits on each side of the fish using a sharp knife.
Season with salt on each side.
Stuff the lemongrass inside the fish.
Place the fish on a plate and steam, until the flesh flakes down to the bone when pulled with a fork, 15 to 18 minutes.
While the fish is steaming, make the sauce. In a small saucepan, add the water, chicken stock cube and Thai palm sugar.
Bring the mixture to a boil over medium heat.
Remove the pot from heat, and add the garlic, chilies, and cilantro. Let steep for a few minutes.
Season with the fish sauce, to taste. Add the lime juice.
Ladle the sauce over the fish. Serve immediately.