Sauce: place all sauce ingredients into blender or food processor. Quickly process to make a smooth, thick puree, adding a little ice water, if necessary. Taste and adjust seasoning, it should be quite spicy.
Fish: Season lightly with salt and pepper.
Heat the oil in a wide deep pan over medium heat.
Add the fish and fry lightly for one minute on one side; flip the fish and raise the heat to medium high.
Add the broth and half of the green sauce, cover and cook for two minutes, or just until the fish is opaque throughout.
Gently stir in the remaining green sauce.
Taste the broth and adjust the salt if necessary.
Ladle the stew into bowls, sprinkle with the scallions, and serve with lime wedges.